Corporation Tax Compliance for Irish Companies

Corporation Tax Ireland

CT1, preliminary tax and financial statements managed as one process.

Irish trading income is generally taxed at 12.5%. Non-trading income is generally taxed at 25%. The classification matters before the CT1 is prepared. Forti manages the full process — from preliminary tax through to iXBRL filing and relief review.

Check my Corporation Tax deadlines View pricing
Key figures at a glance
Trading income rate
12.5%
Non-trading income rate
25%
CT1 deadline
23rd of month 9 after period-end
Preliminary tax (small co.)
23rd of month 11 within period
Late filing surcharge
5% / 10% of tax due
Daily interest rate
0.0274%
LTD accounting plans from €195 + VAT/month
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Filing method
ROS electronic filing
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Accounts format
iXBRL where required
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Reliefs reviewed
486C, R&D, KDB, losses
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Based in
Sandyford, Dublin 18

This service is for you if

You run an Irish limited company and need the CT1 filed through ROS
You are not certain when preliminary tax is due
Your accounts include both trading and rental, investment or other passive income
The company has grown and may now need iXBRL financial statements
You want Section 486C, R&D, KDB, loss or group relief reviewed before filing
The company has missed a return or preliminary-tax payment
You are switching accountants and want the Revenue records reconciled

Which Corporation Tax rate applies

The rate depends on the nature of the income — not just the company type. The classification must be correct before the CT1 computation begins.

12.5%
Qualifying trading income
The activity must amount to a trade. Incorporation or invoicing alone does not settle the classification.
25%
Non-trading income
Rental and investment income are common examples. Income from an excepted trade also falls here.
Varies
Other treatments
Foreign distributions, capital gains, and large multinationals subject to the 15% Pillar Two minimum each follow their own rules.
Mixed income companies: Where a company has both trading and non-trading income, both rates apply. The split must be correctly calculated in the tax computation — applying 12.5% to all income is one of the most common CT errors.

Registration and accounting periods

CRO incorporation does not complete the Corporation Tax registration. Every deadline that follows depends on the accounting period.

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Register within 30 days of trading
The company must notify Revenue after it begins to trade. CRO incorporation creates the legal entity — it does not open a Revenue record.
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A CT accounting period cannot exceed 12 months
If financial statements cover more than 12 months, separate CT1 returns are required for each tax accounting period.
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The year-end drives every deadline
Preliminary tax, CT1 and iXBRL dates all follow the accounting period used for Corporation Tax. An incorrect year-end creates cascading deadline errors.
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Dormant companies need a status review
Do not assume a nil CT1 is always required or never required. Open Revenue periods and filing obligations must be checked.
View Forti tax registration services →

The main Corporation Tax deadlines

Deadlines depend on company size and accounting period. The preliminary tax date and the CT1 date are separate obligations — both must be calendared.

Company type Preliminary tax date Amount due CT1 and balancing payment
Small company 23rd of month 11 of the accounting period Lower of 100% of prior-period liability or 90% of current liability 23rd of month 9 after period-end
New company (first period) No preliminary tax where first-period CT is below €200,000 Final CT paid with the first CT1 23rd of month 9 after period-end
Large company 23rd of month 6 and 23rd of month 11 First instalment based on prior or current liability, topped up to 90% of current liability 23rd of month 9 after period-end
Short period Rules depend on period length Thresholds and amounts may require annualisation or proportionate treatment A separate CT1 is filed for each period of up to 12 months

A small company is one whose prior-period Corporation Tax liability does not exceed €200,000, after applying Revenue's annualisation rule where the prior period was shorter than 12 months.

December year-end example
A small company with an accounting period ending 31 December 2026 normally pays preliminary tax by 23 November 2026. Its CT1 and balancing payment are due by 23 September 2027.

Preliminary tax needs its own calendar

Preliminary tax is an advance payment with no CT1 form to prompt you. The calculation must begin well before month 11 of the accounting period.

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100% previous-period basis
A small company can use the final liability for the prior accounting period, adjusted where that period was shorter than 12 months.
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90% current-period basis
A small company can estimate the current liability and pay at least 90%, with any permitted top-up completed under Revenue's rules.
Underpayment creates interest from day one. If the preliminary payment is late or insufficient, interest runs at 0.0274% per day from the preliminary-tax due date. A missed November payment continues accruing interest through to the following September CT1 — months before anyone notices.

iXBRL financial statements

iXBRL adds machine-readable tags to the financial statements filed with Revenue. It is part of the CT1 filing — not a separate CRO format.

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Small-company exemption
The exemption can apply where total assets are below €4.4 million, turnover is below €8.8 million and average employees are 50 or fewer. All three must be met.
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Not the same as CRO audit exemption
Revenue's iXBRL size criteria are assessed separately for the relevant CT period. Passing the CRO audit-exemption test does not automatically mean iXBRL is not required.
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Tagging follows final accounts
The statutory accounts and tax adjustments must be complete before the iXBRL file is prepared. There is no shortcut to this sequence.
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Late or rejected filings create wider risk
A missing or invalid iXBRL submission can leave the CT1 filing incomplete and delay relief or repayment processing.

Corporation Tax reliefs worth reviewing before filing

These claims should be addressed before the CT1 is submitted — not after. Claim deadlines and supporting evidence requirements apply to each.

Section 486C Start-Up Relief
Up to €40k CT
Full relief where total CT is €40,000 or less, with marginal relief up to €60,000. Capped by qualifying PRSI — not automatic. Trade must have commenced no later than 31 December 2026.
R&D Corporation Tax Credit
35%
Credit rate is 35% for relevant periods ending generally on or after 31 December 2026. Activities, expenditure, notification and claim evidence must meet the statutory conditions.
Knowledge Development Box
Effective 10% rate
Qualifying profits from patented inventions, copyrighted software and certain certified inventions. Applies to defined qualifying assets — not every type of intellectual property.
Trading losses and group relief
Multiple routes
Current-period, carry-back, carry-forward and group claims each depend on the nature of the loss, ownership conditions and claim deadlines.

Section 486C is not an automatic tax holiday

Many companies assume incorporation triggers the relief. It does not.

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Employer Class A PRSI cap
Qualifying employer PRSI counts up to €5,000 per employee or director and €40,000 overall. No PRSI paid means no relief for that period.
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Director Class S PRSI (from 1 January 2025)
Qualifying Class S PRSI contributions can now count toward the cap, up to €1,000 per individual. A new provision that benefits early-stage companies.
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The trade must qualify
The company must carry on a qualifying new trade that commenced within the statutory period, currently no later than 31 December 2026.
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No qualifying PRSI, no relief
A company with no qualifying salary or PRSI activity has no Section 486C relief for that accounting period, regardless of the CT liability.

What happens when Corporation Tax is late

Late filing and late payment are separate issues — both carry financial consequences.

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Late CT1 surcharge
Filed within two months late: 5% of tax due, capped at €12,695. Filed after two months late: 10% of tax due, capped at €63,485.
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Daily interest on unpaid tax
Interest runs at 0.0274% per day from the relevant due date. Preliminary-tax underpayments can accrue interest from November, months before the September CT1 is filed.
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Relief restrictions
Late filing can restrict certain loss, group and other relief claims in addition to the surcharge. These restrictions apply on top of — not instead of — the financial penalties.
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Revenue intervention risk
Repeated late returns, inconsistent classifications and unsupported relief claims can lead to queries or a wider compliance intervention.
Forti can prepare overdue CT1 returns, but Revenue decides the final interest, surcharge and relief-restriction position. The sooner outstanding returns are addressed, the lower the total exposure.

What Forti manages

The full CT1 process — from registration review through to ROS filing and next-period deadline planning.

Corporation Tax registration review
Trading date, accounting periods, tax-reference details and open Revenue obligations checked before work starts.
Preliminary-tax calculation
Small, new or large-company rules applied and the payment instruction prepared before the due date.
Year-end accounts and tax computation
Financial statements finalised and adjusted from accounting profit to taxable profit.
CT1 and ROS filing
Income, gains, losses, close-company information and relief claims completed and submitted within the agreed scope.
iXBRL preparation
Tagged financial statements prepared and submitted where Revenue requires them.
Relief and claim review
Section 486C, R&D, KDB, losses, group relief and other relevant claims considered before filing.

How Forti handles your Corporation Tax

Five stages — from the initial deadline review to next-year planning.

1
Deadline and Revenue review
We confirm the accounting periods, registration, previous CT1 returns, preliminary tax payments and any open Revenue items before the engagement begins.
2
Accounts completed
Bookkeeping, reconciliations and year-end adjustments are finalised. The tax computation cannot begin until the accounts are complete and reconciled.
3
Tax and relief calculation
Trading and non-trading income separated, losses and gains reviewed, preliminary-tax payments reconciled, and all relevant relief claims assessed before the CT1 is drafted.
4
Return approved and filed
You receive the CT summary and approve it before Forti submits the CT1 and iXBRL file through ROS. Nothing is filed without your sign-off.
5
Next dates recorded
The following preliminary-tax date, CT1 deadline and accounts due date are added to the compliance calendar so nothing is missed in the next period.

What we need from you

Complete bookkeeping records and reconciled bank accounts
Sales, purchase, payroll, VAT and fixed-asset information
Loan, investment, rental and foreign-income records
Details of grants, R&D activity, intellectual property and group companies
Previous CT1 returns, preliminary-tax payments and Revenue correspondence
Supporting schedules for losses, capital allowances and relief claims

Corporation Tax pricing

CT1 filing is included in Forti's limited-company accounting plans. The work depends on the accounts, transaction volume, tax adjustments, iXBRL and relief claims — so a standalone CT1-only fee is quoted individually.

Includes CT1
LTD Starter
Monthly bookkeeping and full annual compliance in one plan.
From €195
+ VAT per month
  • Monthly bookkeeping
  • Annual financial statements
  • CT1 filing through ROS
  • CRO Annual Return (B1)
  • VAT returns
  • iXBRL where required
View full plan details
LTD Growth
Adds a dedicated accountant, regular reviews and management accounts.
From €295
+ VAT per month
  • Everything in LTD Starter
  • Dedicated accountant
  • Quarterly management accounts
  • Regular review meetings
  • Priority response
View full plan details
Standalone or historical CT
CT1-only work, overdue returns, iXBRL corrections or complex relief claims.
Exact quote
Send company number and year-end
  • CT1-only engagements
  • Overdue or amended returns
  • iXBRL corrections
  • Mixed income computations
  • Complex relief claims
Get an exact quote

All fees exclude VAT at 23%. Revenue tax, interest and surcharges are paid separately to Revenue.

Common Corporation Tax mistakes

Watching only the CT1 deadline
Preliminary tax can be due inside the accounting period, months before the CT1. Missing it triggers daily interest from the due date.
Applying 12.5% to every source of income
Rental, investment and other non-trading income is generally taxed at 25%. The split must be calculated correctly in the computation.
Assuming a long first set of accounts means one CT1
A Corporation Tax accounting period cannot exceed 12 months. Accounts covering a longer period require separate CT1 returns.
Using the wrong preliminary-tax basis
Small, new and large companies follow different payment rules. Applying the wrong basis can result in an underpayment and interest exposure.
Treating the iXBRL test like CRO audit exemption
Revenue's size criteria are assessed for the relevant CT period and all three limits must be met. The two tests are separate.
Claiming Section 486C without checking PRSI
The relief is capped by qualifying employer and director PRSI actually paid. No PRSI means no relief — regardless of the CT liability.
Treating every software project as R&D
The work must meet the statutory science or technology and qualifying-expenditure tests. Not all software development qualifies.
Leaving relief review until after filing
Claim deadlines and supporting evidence requirements apply before the CT1 is submitted. Leaving it until after filing is often too late.

Related Forti services and guides

Year-end accounts
Finalise the statutory accounts that support the CT1 and iXBRL filing.
View year-end accounts →
Annual compliance
Coordinate financial statements, CT1 and the CRO Annual Return.
View annual compliance →
Limited company accounting
Monthly bookkeeping and annual company compliance under one plan.
View company accounting →
Tax registration
Register for Corporation Tax and other Revenue tax heads.
View tax registration →
Online bookkeeping
Keep records ready for preliminary-tax estimates and year-end accounts.
View online bookkeeping →
Tax reliefs guide
Forti's guide to Irish business tax credits and reliefs.
Read the tax reliefs guide →

Common questions about Corporation Tax

What is the Corporation Tax rate in Ireland?
Qualifying trading income is generally taxed at 12.5%. Non-trading income and income from an excepted trade are generally taxed at 25%.
When is the CT1 due?
The CT1 and balancing payment are due nine months after the accounting period ends, no later than the 23rd day of that month for electronic filing and payment.
When is preliminary Corporation Tax due?
A small company normally pays by the 23rd of month 11 of the accounting period. Large companies can have instalments in months 6 and 11.
What is a small company for preliminary tax purposes?
A company whose prior-period Corporation Tax liability does not exceed €200,000, after annualising a short prior period where required.
Does a new company pay preliminary tax in its first period?
Not where its first-period Corporation Tax liability is below €200,000. It pays the final liability when the first CT1 is filed.
What happens if preliminary tax is late?
Interest runs from the preliminary-tax due date on the unpaid or underpaid amount at 0.0274% per day.
What happens if the CT1 is late?
A 5% surcharge applies within two months, capped at €12,695. A 10% surcharge applies after two months, capped at €63,485. Certain reliefs can also be restricted.
What are the iXBRL exemption limits?
Total assets must be below €4.4 million, turnover below €8.8 million and average employees 50 or fewer. All three criteria must be satisfied for the relevant period.
What is the R&D Corporation Tax Credit rate?
The rate is 35% for relevant accounting periods ending generally on or after 31 December 2026. The activity and expenditure must meet the statutory conditions.
How does Section 486C Start-Up Relief work?
Full relief can apply where total Corporation Tax is €40,000 or less, with marginal relief up to €60,000. The claim is capped by qualifying employer Class A PRSI and eligible director Class S PRSI. No qualifying PRSI means no relief for that period.
What is the Knowledge Development Box rate?
Qualifying profits can be taxed at an effective 10% rate. The relief applies to defined qualifying assets — not every type of intellectual property.
Does a CT accounting period always last 12 months?
A Corporation Tax accounting period cannot exceed 12 months. Accounts covering a longer period require more than one CT1, one for each 12-month period.

Track the payment date, not just the return date

Corporation Tax compliance starts with preliminary tax — before the CT1. Forti keeps the accounting period, preliminary tax, financial statements, iXBRL and relief claims on one calendar.

Check my Corporation Tax deadlines