The CT1 deadline: when your company's corporation tax return is due
Your company's CT1 is due 9 months after its year end, by the 23rd of that month when it's filed and paid through ROS. For a 31 December year end, that's 23 September the following year.
The rule
Every Irish company files a Form CT1 corporation tax return with Revenue for each accounting period, which is usually its financial year. The return is due 9 months after the end of the period, no later than the 21st day of that ninth month. Almost every company files and pays through Revenue Online Service (ROS), which moves the date to the 23rd. Any corporation tax still owed is paid by the same date.
| Year ends | CT1 due (filed and paid on ROS) |
|---|---|
| 31 January | 23 October |
| 28 February | 23 November |
| 31 March | 23 December |
| 30 April | 23 January |
| 31 May | 23 February |
| 30 June | 23 March |
| 31 July | 23 April |
| 31 August | 23 May |
| 30 September | 23 June |
| 31 October | 23 July |
| 30 November | 23 August |
| 31 December | 23 September |
Preliminary tax comes first
Most companies also pay preliminary tax before the year ends. A small company pays it in one go, by the 23rd of the month before its year end. Our preliminary tax guide explains how much and when.
What filing late costs
- A surcharge of 5% of the tax due, up to €12,695, if the return is less than 2 months late.
- A surcharge of 10%, up to €63,485, if it's 2 months late or more.
- Restrictions on reliefs such as trading losses and capital allowances.
- Interest on any tax paid late.
An unfiled return also stops the company getting a tax clearance certificate, which banks, grant bodies and public tenders often ask for.
How to make the deadline comfortably
Get your books finished and your trial balance to your accountant at least 3 months before the CT1 date. That leaves time to prepare the accounts, answer questions, approve them and plan for the tax bill.
Last checked 13 November 2026. This guide is general information, not advice for your company. Tax rules change, so check with us or Revenue before acting.