What is Form CT1 in Ireland?

## What Is Form CT1 in Ireland?

Form CT1 is the annual corporation tax return that every Irish-resident company must file with Revenue. It declares the company's trading profits and losses, calculates the corporation tax liability, and claims any reliefs, credits, or capital allowances available to reduce the bill.

## Filing Deadline

For online filers using ROS, the CT1 is due on the 23rd day of the ninth month after the company's accounting year-end.

• **Year-end 31 December:** CT1 due by 23 September of the following year
• **Year-end 31 March:** CT1 due by 23 December of the same year
• **Year-end 30 June:** CT1 due by 23 March of the following year

This is a filing deadline, not a payment deadline. Balance of tax is due at the same time as the CT1 filing. Preliminary tax must have been paid during the accounting period itself.

## Who Must File a CT1?

Every company resident in Ireland must file a CT1, including:

• Trading companies (whether profitable or loss-making)
• Dormant companies that have been active at any point
• Companies with only rental or investment income
• Companies in liquidation

A non-resident company with an Irish branch or agency must also file a CT1 for the profits attributable to the Irish branch.

## What the CT1 Contains

The CT1 return captures:

## Balance of Tax Payment

The balance of tax is the difference between your total corporation tax liability and any preliminary tax already paid. This balance is due when the CT1 is filed.

Example: A company with a year-end of 31 December 2025 has a total corporation tax liability of €30,000. It paid preliminary tax of €22,500 (75% of liability). The balance of €7,500 is due by 23 September 2026, together with the CT1 filing.

## R&D Tax Credits

The CT1 is where R&D tax credits are claimed. Ireland's R&D regime provides a 25% volume credit on qualifying R&D expenditure, in addition to the deduction already received for the expenditure.

For a company spending €100,000 on qualifying R&D:

• Corporation tax deduction at 12.5%: saves €12,500
• R&D tax credit at 25%: saves a further €25,000
• **Total tax saved: €37,500** on €100,000 of R&D spend

From 1 January 2024, unused R&D credits are fully refundable over three years, regardless of whether the company has a tax liability. This makes the R&D regime highly attractive for pre-profit companies investing in product development.

## Losses and Group Relief

• **Trading losses** in a year can be set against other trading income in the same year, carried forward to offset future profits, or carried back one year against profits in the preceding period
• **Group relief** allows a profitable company in an Irish group to absorb losses from a loss-making fellow group member, reducing the group's overall corporation tax bill

## Late Filing Surcharges

Filing the CT1 after the deadline triggers surcharges:

• **5% of the tax liability** if filed within two months of the deadline
• **10% of the tax liability** if filed more than two months late

These surcharges are in addition to interest on unpaid tax at 0.0219% per day.

## How to File

• Log in to **Revenue Online Service (ROS)** with your company's ROS digital certificate
• Select **File Returns** and choose Corporation Tax (CT1)
• Select the accounting period for the return
• Complete the return using the figures from your audited or certified accounts
• Claim all reliefs, credits, and capital allowances
• Submit the return and pay any balance of tax by the deadline