The Irish Corporation Tax Deadline Calendar: CT1, Preliminary Tax and What Falls Where
Your CT1 is due nine months after year end — but preliminary tax is due before the year even closes. The Irish corporation tax deadline calendar, explained.
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- Abbey Blue Formations
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- 7 min read
Ireland's corporation tax calendar contains one feature that catches out almost every first-time director, and particularly anyone arriving from the UK.
You pay before you file. Preliminary tax falls due before your accounting period has even ended. The CT1 return and the balance of tax follow nine months after it closes. Those two obligations run on entirely separate clocks, and it is perfectly possible to file your CT1 exactly on time and still owe interest.
Here is the full sequence.
The Two Obligations
Preliminary tax for a small company in Ireland — paid before year end
An advance payment against the corporation tax liability for the accounting period that is still running.
For a small company — defined for this purpose as one whose corporation tax liability in the preceding accounting period was less than €200,000 — preliminary tax is paid in a single instalment, due 31 days before the end of the accounting period and before the 23rd of that month.
To avoid interest, a small company pays either 90% of the current period's liability or 100% of the prior period's liability. Most opt for the prior-year figure because it is a known number rather than an estimate.
Large companies — those with a prior-period liability of €200,000 or more — pay in two instalments where the accounting period is longer than seven months. The first is due on the 23rd of the sixth month of the period, at either 50% of the previous period's liability or 45% of the current period's. The second is due on the 23rd of the eleventh month, bringing preliminary tax up to 90% of the final liability.
The CT1 return — filed nine months after year end
The company files its return and pays any balance of tax nine months after the end of the accounting period, on or before the 23rd of that ninth month when filing and paying electronically through ROS.
Every Irish company must file a CT1 annually, whether or not it traded and whether or not it made a profit. A company with no activity files a nil return.
A Worked Calendar: 31 December Year End
For the most common year end, the sequence runs like this:
| Date | What is due |
|---|---|
| 23 November 2026 | Preliminary tax for the year ending 31 December 2026 (small company, single instalment) |
| 31 December 2026 | Accounting period ends |
| 23 September 2027 | CT1 return for 2026 and balance of tax |
| 23 November 2027 | Preliminary tax for the year ending 31 December 2027 |
Notice what that means in practice: in November 2026 you are paying tax on a year that has not finished, and in September 2027 you are settling the balance on it. Two payments on the same year, ten months apart.
If your year end is not December
Apply the pattern rather than the dates. Preliminary tax lands 31 days before your period end; the CT1 lands nine months after it. A 30 June year end means preliminary tax in May and the CT1 the following March.
What Late Costs You
There are two separate penalties, and you can incur both.
Late payment interest
Interest runs at a daily rate of 0.0219% — roughly 8% a year — on tax paid late or not paid in full. It runs from the original due date, which is why a shortfall in preliminary tax generates interest even where the CT1 is filed perfectly on time.
Interest charged cannot be appealed to the Tax Appeals Commission and cannot be reduced once charged.
Late filing surcharge
A surcharge applies on top of the tax due where the return is late: 5% of the tax due, capped at €12,695, where the return is filed within two months of the filing date, rising for longer delays. There is also a restriction on losses and certain reliefs claimed on a late return.
The surcharge is calculated on the tax due, not on the amount outstanding — so a company that has paid its tax in full but files the return late can still incur it.
The Other Filings That Share the Date
The CT1 is not the only thing due on that ninth-month deadline. Form 46G, the third-party payments return, shares it — and it is routinely forgotten by companies focused on the CT1 itself.
Separately, and often confused with all of this, your CRO annual return runs on a completely different cycle from your tax obligations. Missing it has its own consequences, covered in our post on audit exemption.
Practical Advice
- Set the preliminary tax reminder for two months before year end, not at year end. You need time to estimate.
- Use the prior-year basis if it is available and affordable. Paying 100% of last year's liability is certain; estimating 90% of this year's is not.
- Keep the bookkeeping current. Both dates depend on knowing your numbers, and a company scrambling to reconstruct a year of records in September has already lost. Our bookkeeping packages exist for this.
- Do not confuse tax registration with tax filing. Registering for corporation tax and VAT is a one-off step at formation; the calendar above runs every year afterwards.
Frequently Asked Questions
When is my CT1 return due in Ireland?
Nine months after the end of your accounting period, on or before the 23rd day of that ninth month when you file and pay through ROS. For a 31 December year end, that means 23 September the following year. The balance of any corporation tax owed is due on the same date. Paper filers face an earlier deadline of the 21st, though effectively all Irish companies file electronically.
What is preliminary corporation tax and when do I have to pay it?
Preliminary tax is an advance payment towards the corporation tax for an accounting period that is still running. A small company — one whose corporation tax liability in the preceding period was under €200,000 — pays it in a single instalment due 31 days before the end of the accounting period, and before the 23rd of that month. This surprises people arriving from jurisdictions where tax is paid only after year end. For a December year end, the payment falls in November, before the year has closed.
How much preliminary tax do I have to pay to avoid interest?
A small company pays either 90% of the corporation tax liability for the current accounting period or 100% of the liability for the preceding period. Most companies choose the prior-year basis because it is a known figure and removes the risk of underestimating. Large companies pay in two instalments on a different basis. If you underpay, interest runs from the original due date at a daily rate of 0.0219%, roughly 8% a year, even if you file your CT1 on time.
What happens if I file my corporation tax return late in Ireland?
A surcharge applies on top of the tax due — 5% of the tax due, capped at €12,695, where the return is filed within two months of the filing date, with a higher rate for longer delays. Losses and certain reliefs claimed on a late return can also be restricted. Note that the surcharge is based on the tax due rather than on any unpaid balance, so a company that has already paid its tax in full can still be surcharged for filing the return late.
Do I need to file a CT1 return for a company with no profit or no trade?
Yes. Every Irish company files a CT1 annually regardless of whether it traded or made a profit. A company with no activity files a nil return. Companies frequently assume that no profit means no obligation, and it is one of the more common ways an otherwise well-run dormant entity ends up with a compliance problem.
What is the difference between my CT1 deadline and my CRO annual return deadline?
They are entirely separate obligations to two different bodies on two different cycles. The CT1 goes to Revenue nine months after your accounting period ends. The annual return goes to the Companies Registration Office on your own annual return date, and missing it brings late filing penalties and can affect audit exemption. Neither satisfies the other, and having one filed on time tells you nothing about the other.
Can I get interest on late corporation tax reduced or appealed?
No. Interest charged on late or underpaid corporation tax cannot be appealed to the Tax Appeals Commission, and once charged, the full amount is payable and cannot be reduced. This is a meaningful difference from some other charges and is a good reason to pay preliminary tax on the safer prior-year basis rather than on an optimistic estimate.
I have a 31 December year end — what dates should be in my diary?
Preliminary tax by 23 November of the year in question. CT1 return and balance of tax by 23 September of the following year. Form 46G shares the September date. Then preliminary tax for the next year falls due that November — meaning two corporation tax payments within about two months of each other in the autumn, on two different years. Planning cash flow around that overlap is what most first-year companies miss.