CT1 Return and Corporation Tax Deadlines in Ireland: When to File, When to Pay and What Late Filing Costs

Corporation tax deadlines are the most common source of avoidable cost for Irish companies. The tax itself is usually paid in full. The problem is timing: preliminary tax paid late or underpaid attracts interest, and a CT1 filed late attracts a surcharge and a restriction on loss relief that delays when losses can be used.

This guide sets out the three dates every company has to manage, the rules for small and large companies, a worked calendar for the most common year end, and exactly what happens when a date is missed. Every rule below comes from Revenue's Tax and Duty Manuals or the Department of Finance's Finance (No. 2) Bill 2026 publications, linked at the end. For how the tax itself is calculated, see our guide to corporation tax in Ireland.

The three dates every company has

For each accounting period, a company has:

1. A preliminary tax payment date, before the period ends.
2. A CT1 filing date, nine months after the period ends.
3. A balance payment date, which is the same as the filing date.

Companies that are part of a group or that pay income tax on certain payments have additional obligations, but those three dates cover the vast majority of Irish companies.

The 23rd of the month rule

Most of the statutory dates are expressed as a day in a month, for example 31 days before the period ends or nine months after it ends. The legislation then brings the date forward to the 21st of that month, or the 23rd where the company files and pays through ROS. Since almost every company is required to file electronically, the 23rd is the date that matters in practice.

The rule works as "the earlier of" two dates. Where the statutory day falls on or after the 23rd, the deadline moves to the 23rd. Where it falls before the 23rd, the statutory day itself is the deadline. A company with a 4 January year end, for example, has a statutory balance date of 4 October, which is earlier than 23 October, so 4 October applies.

Preliminary tax for small companies

A small company for this purpose is one whose corporation tax liability for the previous accounting period was €200,000 or less. Close company surcharges count towards that figure. Where the previous period was shorter than twelve months, the limit is reduced in proportion. The Finance (No. 2) Bill 2026, published on 19 October 2026, raises this threshold to €350,000 for accounting periods beginning on or after 1 January 2027. The Bill is not yet law.

A small company makes one preliminary tax payment. It can choose to pay either:

• 100% of its corporation tax liability for the previous accounting period, or
• 90% of its corporation tax liability for the current accounting period.

The payment is due on the earlier of 31 days before the end of the accounting period or the 23rd of that month. For a twelve-month period ending on 31 December, that means 23 November.

The 100% prior-year option is the safe harbour. If the company pays 100% of last year's liability on time, it has met its obligation even if this year's profits turn out to be much higher. The 90% current-year option saves cash in a declining year but requires an accurate forecast with more than a month of the year still to run. If the final liability comes in higher than expected and the payment turns out to be less than 90%, interest runs from the preliminary tax date on the shortfall.

Preliminary tax for large companies

A large company is one whose corporation tax liability for the previous accounting period exceeded €200,000. It pays preliminary tax in two instalments.

The first instalment is due on the earlier of the last day of the sixth month of the accounting period or the 23rd of that month. The amount is either 50% of the previous period's liability or 45% of the current period's liability.

The second instalment is due on the earlier of 31 days before the end of the period or the 23rd of that month. It must bring the total paid up to 90% of the current period's liability.

A company whose accounting period is shorter than seven months pays 90% in a single instalment.

The Finance (No. 2) Bill 2026 also adds a top-up option for accounting periods beginning on or after 1 January 2027. Where a company has paid at least 80% of its final liability by its preliminary tax date (or, for a large company, its second instalment date), it can pay the balance within four months of the period end and be treated as having met that preliminary tax obligation. The Bill is not yet law.

New companies

A new company does not have to pay preliminary tax for its first accounting period, provided its liability for that period is under €200,000. It simply pays the full liability when it files its first CT1. This is a genuine cash-flow advantage in year one, but it also means the first balance payment and the second year's preliminary tax can land close together, so it is worth planning for. The Finance (No. 2) Bill 2026 raises the €200,000 limit to €350,000 for accounting periods beginning on or after 1 January 2027.

Worked calendar: year end 31 December 2026

For a small company with a 31 December 2026 year end that files and pays through ROS:

• 23 November 2026: preliminary tax due. Pay either 100% of the 2025 liability or 90% of the estimated 2026 liability.
• 23 September 2027: CT1 for 2026 due, together with the balance of corporation tax for 2026.
• 23 December 2027: latest date for iXBRL financial statements, where required, under Revenue's practice of allowing them up to three months after the CT1 due date. Filing them with the CT1 is safer, for the reasons below.

For a large company with the same year end:

• 23 June 2026: first instalment, being 50% of the 2025 liability or 45% of the estimated 2026 liability.
• 23 November 2026: second instalment, bringing the total to 90% of the 2026 liability.
• 23 September 2027: CT1 and balance.

If the year end is not 31 December, the same pattern applies to whichever months are 31 days before the period end and nine months after it. The dates for every month end are listed on our tax calendar.

What happens if preliminary tax is underpaid or late

Where preliminary tax is insufficient or paid late, the whole liability for the period is treated as having been due on the preliminary tax date (for a large company, 45% on the first instalment date and the balance on the second), and interest runs from that date until the tax is paid. Revenue's worked examples use a daily rate of 0.0219%, which is roughly 8% a year.

For accounting periods beginning on or after 1 January 2027, the Finance (No. 2) Bill 2026 changes this so that the amount treated as due on the first instalment date is the lower of 45% of the current liability and 50% of the previous period's liability.

In Revenue's example, a small company with a €125,000 liability pays €100,000 on 23 November and the remaining €25,000 the following September. Because €100,000 is less than both 90% of the current year and 100% of the prior year, interest runs on the €25,000 for the full ten months. Even where the right amount is paid but a few weeks late, interest runs on that payment for those weeks, and the balance paid with the CT1 also carries interest back to the preliminary tax date, because the obligation was not met on time.

Interest also arises retrospectively. If a Revenue audit or a voluntary disclosure increases the liability for a past period, the company has to go back and test whether its preliminary tax for that period still met the 90% or 100% test. Often it does not, and interest is recalculated from the original preliminary tax date.

What happens if the CT1 is late

Two separate consequences follow a late CT1.

The first is the late filing surcharge under section 1084. The tax liability for the period is increased by 5% where the return is filed within two months of the deadline, capped at €12,695, or by 10% where it is more than two months late, capped at €63,485. The surcharge applies even if the tax was paid in full and on time. A deliberately or carelessly incorrect return is treated as a late return unless it is corrected before the filing date. Where Revenue charges a penalty for the error instead, the surcharge does not also apply.

The second is the restriction of reliefs under section 1085, and this is the one that catches companies with losses. Where the return is less than two months late, claims for loss relief, excess capital allowances and group relief are cut by 25%, capped at €31,740 for the period. Where it is two months or more late, the cut is 50%, capped at €158,715.

A loss-making company that thinks there is nothing to lose by filing late can find it cannot use up to half of that year's loss against other profits or in a group claim. The restricted amount is not usually lost, but it can only be carried forward to later years, which delays the tax saving.

Revenue's manual also makes the point that a CT1 which omits a close company surcharge or the income tax due on a loan to a shareholder is an incomplete return, which opens the door to interest and a surcharge for failing to submit a correct return.

iXBRL accounts

Larger companies must also file their financial statements in iXBRL format. A company is excluded if it meets all three of these criteria: a balance sheet total (gross assets, before deducting liabilities) of no more than €4.4 million, turnover of no more than €8.8 million, and an average of no more than 50 employees. Inactive companies and insolvent companies in liquidation are also generally excluded, while companies dealt with by Revenue's Large Corporates Division or High Wealth and Financial Services Division must file iXBRL whatever their size. Excluded companies complete the extracts from accounts section of the CT1 instead.

Revenue's current practice allows iXBRL accounts to be filed with the CT1 or up to three months after the CT1 due date. Until they are filed, the CT1 is treated as incomplete, which can hold up refunds and block a tax clearance application, so filing them with the CT1 is the safer course. Where the ROS upload sticks at "processing" past the deadline, Revenue will consider waiving any surcharge if the company contacts the ROS helpdesk and raises the issue through MyEnquiries with evidence.

How the CT1 is filed

The CT1 is filed on ROS. In outline:

1. Finalise the accounts and the corporation tax computation, including capital allowances, loss claims, any close company surcharge and any start-up relief or R&D credit claim.
2. Log in to ROS, select the company, and open the CT1 for the accounting period.
3. Complete the panels for trading results, other income, chargeable gains, capital allowances, losses, close company details and reliefs claimed. The figures must match the computation.
4. Upload the iXBRL financial statements where required.
5. Review the calculated liability against the computation, then sign and submit.
6. Pay the balance through ROS by the same date, usually by ROS Debit Instruction or a Single Debit Instruction for a once-off payment from the company's bank account. Revenue no longer accepts commercial credit or debit cards.

The ROS form is unforgiving of inconsistencies between panels. The most common errors we see on returns prepared in-house are rental income entered as trading income (taxed at 12.5% instead of 25%), capital items claimed twice (as an expense and again through capital allowances), and close company surcharge panels left blank.

Getting help with your CT1 and corporation tax

We prepare CT1 returns for Irish companies, calculate preliminary tax under both the prior-year and current-year methods so you can choose the better one, and diarise every date so you are reminded before each one. Contact us today if you need help with CT filings, or get in touch here and we will get back to you.

Sources

The rules and figures in this guide are taken from these Revenue publications:

• Tax and Duty Manual Part 41A-07-02, Payment of Preliminary Corporation Tax
• Tax and Duty Manual Part 47-06-08, Surcharge for Late Submission of Returns
• Tax and Duty Manual Part 47-06-04, Late Submission of Corporation Tax Returns and Restriction of Reliefs
• Tax and Duty Manual Part 47-06-01, Return Filing Dates and iXBRL
• Tax and Duty Manual Part 41A-03-01, Submission of iXBRL Financial Statements
• Notes for Guidance on Part 47 TCA 1997 (Section 1085)
• Online Payment Methods for Tax

Information provided courtesy of the Revenue Commissioners under a Creative Commons Attribution 4.0 International (CC BY 4.0) licence. The 2027 preliminary tax changes are from section 74 of the Department of Finance's Explanatory Memorandum to the Finance (No. 2) Bill 2026.