Form CT1 Explained: Corporation Tax Filing for Irish Companies
- Aleksandar Davidov
- Jul 2
- 2 min read
If you operate an Irish Limited Company, understanding Form CT1 is essential to staying compliant with Revenue and avoiding penalties. Whether you’re a startup, a growing SME, or a UK-based director managing an Irish subsidiary, this guide breaks down what you need to know about CT1, Ireland’s annual corporation tax return.
What is Form CT1?
Form CT1 is the annual Corporation Tax return filed by Irish companies to Revenue. It declares your company’s taxable profits, calculates tax due, and includes supplementary information like director details, capital allowances, R&D credits, and more.
It must be filed online via Revenue Online Service (ROS).
Who Needs to File CT1?
All Irish-resident companies are required to file Form CT1 annually — even if:
You made no profit
You had no trading activity
You’re a dormant company
Foreign companies with a branch in Ireland may also be required to file.
CT1 Filing Deadline
Your CT1 is due 9 months after your accounting year-end, but no later than the 23rd day of that month if filing via ROS.
Example:Â If your year-end is 31 December 2024, your CT1 is due by 23 September 2025.
Missing the deadline leads to surcharges of 5% or 10%, and possible restriction of reliefs.
What Information Goes Into Form CT1?
Your CT1 return includes:
Company name, CRO number, and tax reference
Director names and PPSNs
Profit & loss breakdown
Adjusted net profit for tax purposes
Capital allowances & depreciation
Losses carried forward
Tax due, credits claimed (e.g. R&D, foreign tax credit)
Preliminary tax paid
It’s common to attach your iXBRL-tagged financial statements (mandatory for many companies).
What is Preliminary Tax?
Before filing the CT1, you must make a Preliminary Tax payment for the financial year. This acts as a down payment on your expected Corporation Tax liability.
Due Date: No later than 1 month before your year-end, by the 23rd of that month.
If not paid correctly, you may face interest charges.
Penalties for Late CT1 Filing
Revenue applies strict penalties for non-compliance:
5% surcharge if late <2 months
10% surcharge if later
Loss of tax reliefs (e.g. R&D, loss carryforwards)
Possible Revenue audit trigger
How We Help
As a UK-qualified accountancy firm with deep experience in Irish tax, we offer:
Complete CT1 preparation & submission via ROS
Preliminary tax calculations
iXBRL-tagged financial statements
Cross-border group tax support (UK/IE)
Do not hesitate to contact us:
📞+44 7939 952432