Corporation Tax Deadlines Ireland: Key Dates and Filing Requirements
Missing a corporation tax deadline can lead to penalties, interest, and restricted reliefs.
However, many companies are unclear on when corporation tax must be paid and when returns must be filed.
In Ireland, corporation tax operates on a self-assessment basis. This means companies must calculate their own tax, make payments on time, and file returns within strict deadlines.
In this guide, we explain the key corporation tax deadlines in Ireland, including payment dates, filing requirements, and what happens if deadlines are missed.
Why Corporation Tax Deadlines Matter
Meeting deadlines is not just about compliance.
In practice, late filing or late payment can:
- Trigger surcharges and interest
- Delay tax refunds
- Restrict the use of loss relief
- Create cash flow pressure
Therefore, understanding deadlines is an important part of managing your company’s tax position.
Key Corporation Tax Deadlines in Ireland
Irish companies must meet two main obligations:
- Pay corporation tax
- File a corporation tax return (CT1)
1. Filing the CT1 Return
Companies must file a CT1 return for each accounting period.
The deadline is:
- 9 months after the end of the accounting period, and
- No later than the 23rd day of that month
For example:
- Accounting period ends: 31 December 2025
- Filing deadline: 23 September 2026
Companies must file through the Revenue Online Service (ROS).
2. Paying Corporation Tax
Corporation tax is paid in two stages:
- Preliminary tax
- Balance payment
Preliminary Corporation Tax Explained
Preliminary tax is a payment made before the final corporation tax liability is confirmed.
However, the rules differ depending on whether a company is classified as small or large.
Small Companies
A small company is one whose corporation tax liability in the previous accounting period was €200,000 or less.
Small companies:
- Pay preliminary tax in one instalment
- Pay it by the 23rd day of the 11th month of the accounting period
For example:
- Year-end: 31 December 2025
- Preliminary tax due: 23 November 2025
The amount paid is typically:
- 90% of the current year liability, or
- 100% of the prior year liability
Large Companies
A large company is one whose corporation tax liability in the previous period exceeded €200,000.
Large companies must:
- Pay preliminary tax in two instalments
In general:
- The first instalment is due in month 6
- The second instalment is due in month 11
Together, these payments must cover at least 90% of the current year’s liability
New Companies
New or start-up companies may not have to pay preliminary tax in their first accounting period if their corporation tax liability is below €200,000.
Instead, they pay their full corporation tax when filing the CT1 return.
Balance of Corporation Tax
Once the accounting period ends, companies calculate their final corporation tax liability.
If preliminary tax does not cover the full amount, the balance must be paid by:
- The same date as the CT1 filing deadline
Therefore, both the final payment and tax return are due together.
Consequences of Missing Deadlines
Failing to meet corporation tax deadlines can have several consequences.
Surcharges
Late filing can result in a surcharge on the corporation tax liability.
The surcharge depends on how late the return is submitted.
Interest Charges
Late payment of corporation tax leads to interest charges.
These apply from the due date until payment is made.
Restriction of Loss Relief
Late filing can also restrict the amount of loss relief a company can claim.
This can directly affect how losses are used. You can read more in our guide to Corporation Tax Losses in Ireland.
Common Mistakes Companies Make
In practice, many companies:
- Miss preliminary tax deadlines
- Underestimate tax liabilities
- Leave calculations too late
- Confuse filing deadlines with payment deadlines
- Rely on year-end accounts instead of ongoing tracking
However, most of these issues can be avoided with proper planning.
How This Links to Tax Planning
Corporation tax deadlines are closely linked to broader tax planning.
For example:
- Loss relief claims must be made within specific time limits
- Group relief requires correct timing and coordination
You can also read our guide to Group Relief in Ireland to understand how losses can be used across companies.
How We Help with Corporation Tax Compliance
At Richard OShea Consultancy, we support businesses with ongoing tax compliance and planning.
This includes:
- Monitoring key corporation tax deadlines
- Calculating preliminary tax accurately
- Preparing and filing CT1 returns
- Identifying opportunities for loss relief and tax planning
For many clients, this forms part of our Monthly Accounting Services in Ireland, where financial performance and tax obligations are reviewed throughout the year.
As a result, businesses can avoid last-minute pressure and reduce the risk of errors.
Final Thoughts
Corporation tax deadlines in Ireland follow a structured timeline. However, they require careful attention.
Companies must:
- Pay preliminary tax during the year
- File their CT1 return on time
- Pay any balance of tax due
Missing deadlines can result in penalties, interest, and reduced tax relief.
If you are unsure about your company’s deadlines or obligations, reviewing your position early can help avoid unnecessary costs and ensure full compliance.
This article is intended for informational purposes only and should not be considered a replacement for professional advice. The author(s) disclaim any liability for actions taken or not taken based on the content of this document. It is recommended to seek tailored advice before making any decisions related to the topics discussed in this article.
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