Corporation Tax in Ireland: CT1 Deadline, Preliminary Tax and What Small Companies Must File

Staxo Team, 2026-09-16

Once your Irish limited company is trading, corporation tax becomes an annual cycle: pay preliminary tax before the year ends, file the CT1 nine months after it ends, and attach financial statements in the right format. The rates are low by international standards, but the deadlines are strict and the surcharges for missing them are not small.

This guide covers the rates, the CT1 filing date, preliminary tax for small companies, the iXBRL requirement, surcharges, and the close company surcharge that catches owner-managed companies with money left in the bank.

The two corporation tax rates

Revenue applies two rates:

• 12.5% on trading income. This is the rate most small companies pay on their profits from selling goods or services.
• 25% on non-trading income, such as rental and investment income, and on income from an excepted trade such as dealing in land.

A company can have both. A consultancy that also rents out a property pays 12.5% on consultancy profits and 25% on the rent. Capital gains made by the company are also brought into the CT1, at the capital gains tax rate.

When the CT1 is due

Revenue's rule is that a company must file its return and pay any tax due nine months after the end of the accounting period, and on or before the 23rd of that ninth month. Filing is mandatory through ROS.

An accounting period for corporation tax cannot be longer than 12 months. A first set of accounts covering, say, 15 months from incorporation is split into two accounting periods for tax, each with its own CT1.

Preliminary corporation tax

Preliminary tax is paid before the accounting period ends. The rules depend on whether Revenue treats the company as small or large.

Small companies

Revenue defines a small company as one whose corporation tax liability was not above 200,000 euro in the previous accounting period. A small company pays preliminary tax in one instalment, due 31 days before the end of the accounting period and no later than the 23rd of that month. For a 31 December year end, that is 23 November.

The amount must be at least the lower of:

• 100% of the previous accounting period's liability, or
• 90% of the current period's final liability.

Most small companies use the 100% prior year figure because it is known. If profits have fallen, 90% of the current year estimate may be lower, but the estimate has to be right. Paying too little means interest from the due date.

New companies

Revenue's guidance says a new or start-up company does not have to pay preliminary tax for its first accounting period if its liability for that period is under 200,000 euro. It pays the full amount with the first CT1.

Large companies, briefly

A company whose liability exceeded 200,000 euro in the previous period pays in two instalments. The first is due on the 23rd of the sixth month of the period and must be 50% of the previous period's liability or 45% of the current one. The second is due on the 23rd of the eleventh month and must bring the total to 90% of the current period's liability. If the period is shorter than seven months, 90% is paid in a single instalment.

Financial statements in iXBRL

The CT1 does not stand alone. Most companies must also upload their financial statements to ROS in iXBRL, a tagged electronic format that Revenue can read automatically. Revenue exempts a company from iXBRL only if all three of the following apply:

• total assets, before deducting liabilities, are under 4.4 million euro
• turnover is under 8.8 million euro
• average employees are 50 or fewer

A company that meets all three does not have to file iXBRL statements, though it still enters the accounts figures on the CT1 itself. A company that fails any one of the three must file iXBRL. Most accounting software and accountancy firms can produce the tagged file. Revenue's guidance sets a time limit for the iXBRL upload that runs from the CT1 due date; check the current position on revenue.ie before relying on it.

Surcharges and restrictions for late filing

Revenue applies the same surcharge scale to companies as to individuals:

• 5% of the tax due, up to a maximum of 12,695 euro, if the CT1 is filed within two months of the deadline.
• 10% of the tax due, up to a maximum of 63,485 euro, if it is filed more than two months late.

Revenue also restricts certain reliefs when a return is late, including loss relief, group relief and excess capital allowances. A loss-making company that files late can therefore lose part of the loss it hoped to carry forward. Interest is charged on late payment for each day past the due date.

The close company surcharge

Most Irish owner-managed companies are close companies, meaning they are controlled by five or fewer people or by their directors. Revenue applies an extra charge to close companies that hold on to certain income rather than paying it out. A surcharge of 20% applies to undistributed after-tax estate and investment income, such as rent and deposit interest. A close service company, which is broadly one whose income comes from professional services such as accountancy, architecture, engineering or consultancy, is also liable to a 15% surcharge on one half of its undistributed trading income. The surcharge is avoided or reduced if the income is distributed within 18 months of the end of the accounting period in which it arose. If your company earns professional fees or investment income, ask a qualified accountant to check the position before the 18 months run out.

Corporation tax checklist for a small company

• Note the accounting period end and calculate the two dates: preliminary tax (23rd of the twelfth month) and CT1 (23rd of the ninth month after year end).
• Register for corporation tax on ROS within the required time after starting to trade.
• Estimate the year's profit by month ten so the preliminary tax basis can be chosen.
• Pay preliminary tax through ROS by the 23rd of the last month of the period.
• Have the financial statements finalised within six months of year end, which also serves the CRO annual return.
• Check whether iXBRL applies using the three thresholds.
• File the CT1, upload iXBRL if required, and pay the balance by the 23rd of the ninth month.
• Review close company surcharge exposure before the 18 month window closes.
• Keep records for six years.

Frequently asked questions

My company made a loss. Do I still file a CT1?

Yes. Every company within the charge to corporation tax files a return for each accounting period, including loss-making and dormant companies. Filing on time also protects your right to carry the loss forward without restriction.

Is the deadline the 21st or the 23rd?

The statutory date is the 21st. Revenue extends it to the 23rd when the return is filed and paid through ROS. Since ROS filing is mandatory for companies, the 23rd is the date that applies in practice.

Can a director's salary reduce corporation tax?

Salary paid through payroll is a deductible expense of the company, so it reduces the profit charged at 12.5%. The director then pays income tax, USC and PRSI on it personally. Whether salary, pension contributions or retained profit is better depends on your circumstances; take advice from a qualified accountant.

What happens if I pay preliminary tax late?

Interest runs from the due date on the shortfall between what you paid and the minimum required. There is no surcharge for late preliminary tax alone; the surcharge attaches to a late CT1.

Sources

• Revenue: Corporation Tax, basis of charge and rates
• Revenue: Corporation Tax payment and filing
• Revenue: Preliminary Corporation Tax
• Revenue: When is preliminary CT due?
• Revenue: Who must submit financial statements in iXBRL?
• Revenue: Surcharge on undistributed income of close companies

Staxo gives Irish limited companies accounting software with human support, so the profit figure behind your preliminary tax estimate and your CT1 is one you can trust. See Staxo plans.