If you run a company in Dublin or anywhere else in Ireland, your corporation tax deadlines in Ireland run off your company’s own year end, not the calendar. That is why a December year end and a March year end have such different dates. This guide sets out the CT1 deadline, the dates for preliminary corporation tax, the late filing surcharges and a few worked dates, so you can check your own diary against Revenue’s rules.
Key takeaways
- The CT1 return and any balance of corporation tax are due nine months after your accounting period ends, by the 23rd of the ninth month when you file and pay through ROS.
- A small company (prior-period corporation tax of €200,000 or less) pays preliminary tax once, by the 23rd of the 11th month of the period.
- A large company pays in two instalments: by the 23rd of month 6 and by the 23rd of month 11.
- Filing late costs a 5% surcharge (maximum €12,695) or 10% (maximum €63,485), and interest runs at 0.0219% a day.
- A 31 December 2025 year end had a CT1 date of 23 September 2026; a 31 December 2026 year end has 23 September 2027.
What are the corporation tax deadlines in Ireland?
Corporation tax deadlines in Ireland depend on your accounting period end. The CT1 return and any balance are due nine months after the period ends, by the 23rd of the ninth month on ROS. Small companies pay preliminary tax by the 23rd of the 11th month, and large companies in two instalments.
The CT1 filing date moves with your year end, so a company that changed its accounting date will have different dates from last year. Here are some examples worked from Revenue’s rule:
| Accounting period ends | CT1 and balance due (ROS) | Small-company preliminary tax due |
|---|---|---|
| 31 December 2025 | 23 September 2026 (passed) | 23 November 2025 (passed) |
| 31 March 2026 | 23 December 2026 | 23 February 2026 (passed) |
| 30 June 2026 | 23 March 2027 | 23 May 2026 (passed) |
| 30 September 2026 | 23 June 2027 | 23 August 2026 (passed) |
| 31 December 2026 | 23 September 2027 | 23 November 2026 |
Your return needs finished accounts behind it, so the date that really matters is when your figures are ready. Our annual accounts and CT returns page shows what that involves, and the CT1 itself is filed on ROS. According to Revenue’s corporation tax filing guidance, companies that do not file and pay electronically must pay by the 21st instead.
How does preliminary corporation tax work?
Preliminary corporation tax is the advance payment of your tax bill before the year ends. A small company, with prior-period corporation tax of €200,000 or less, pays once by the 23rd of the 11th month, using either 100% of last period’s tax or 90% of this period’s tax.
Large companies, with prior-period corporation tax above €200,000, work differently:
- If the accounting period is longer than seven months, pay in two instalments. The first is due by the 23rd of month 6 (50% of the prior-period tax or 45% of the current-period tax). The second is due by the 23rd of month 11 and brings the total to 90% of the final tax.
- If the period is seven months or less, pay 90% in one instalment.
- New companies pay no preliminary tax in their first accounting period if the tax is under €200,000. They pay the full amount with the return.
Worked example. A Dublin company has a 31 December 2026 year end. Last year’s corporation tax was €8,000 and this year’s is heading for €12,000. By 23 November 2026 it pays €8,000, which is 100% of last year’s (90% of this year would be €10,800, but the lower route is enough). The remaining €4,000 is due with the CT1 by 23 September 2027. Getting your corporation tax forecast right during the year is what stops that final balance coming as a shock.
What is the penalty for filing a CT1 late?
Revenue charges a surcharge of 5% of the tax due, capped at €12,695, if the CT1 is filed within two months of the deadline. After two months the surcharge is 10%, capped at €63,485. Interest on late or underpaid tax also runs at 0.0219% a day.
Using a company with €20,000 of corporation tax as the example:
- Filed six weeks late: 5% surcharge of €1,000.
- Filed three months late: 10% surcharge of €2,000.
- €5,000 of that tax paid 30 days late: interest of about €32.85 (€5,000 x 30 days x 0.0219%).
Revenue also says late filers may face restrictions on excess capital allowances, loss relief and group relief. Interest cannot be appealed to the Tax Appeals Commission or reduced, so paying promptly matters even when the return is still being finished. If you have missed a date already, file as soon as you can: the surcharge steps up at a fixed point, and every extra week adds interest. In short, the cheapest day to fix a missed CT1 is today.
Do I file the CT1 and the CRO annual return together?
No. The CT1 goes to Revenue through ROS, and the annual return goes to the CRO, so they are separate filings with separate deadlines. Your accounts feed both, which is why limited company accounting that is kept up to date makes both far easier.
Many Dublin start-ups and small companies get caught by this: the CT1 is done, but the CRO date passes unnoticed, or the other way round. If your company is new and you are also learning your duties as a director, our post on tax for company directors covers the personal side. Companies that want their accounts done properly each year can read how filing CT returns online works in practice.
Getting help with corporation tax in Dublin, Galway and across Ireland
Fuchsia Bell is a firm of chartered accountants in Castlemaine, Co. Kerry, run by Gero O’Connor, working with limited companies all over Ireland online. For Dublin companies, a chartered accountant in Dublin can set up a diary of your CT1 and preliminary tax dates. If you are in the west, an accountant in Galway can do the same fully online. We also work with company directors in Mayo and right across Ireland.
Frequently Asked Questions
What is the CT1 filing date for a 31 December year end?
The CT1 filing date is 23 September of the following year when you file and pay through ROS. For a 31 December 2026 year end, that is 23 September 2027. Companies that do not file and pay electronically must pay by the 21st of the ninth month.
When is preliminary corporation tax due for a small company?
A small company pays preliminary corporation tax once, by the 23rd of the 11th month of the accounting period. For a 31 December 2026 year end, that is 23 November 2026. The payment is at least 100% of last period’s tax or 90% of this period’s.
Do start-ups pay preliminary tax in their first year?
No. A new company pays no preliminary tax in its first accounting period if its corporation tax is under €200,000. It pays the full corporation tax with its first return instead, by the 23rd of the ninth month after the period ends.
How much is the late filing surcharge on a CT1?
It is 5% of the tax due, capped at €12,695, if you file within two months of the deadline. After two months it is 10%, capped at €63,485. Late filers may also lose some capital allowance, loss relief and group relief claims.
Is interest charged on late corporation tax?
Yes. Revenue charges interest on late or underpaid corporation tax at 0.0219% a day. It cannot be appealed to the Tax Appeals Commission and cannot be reduced, so paying promptly matters. On €5,000 paid 30 days late, the interest is about €32.85.
If your company’s dates are a bit tangled, talking it through with a chartered accountant can usually sort it out in one conversation. We can confirm your year end, map out your CT1 and preliminary tax dates and tell you what to pay and when.
