Preliminary Tax is the estimate of Income Tax and USC / PRSI payable by you for a tax year and must be paid by 31 October in the year in question.(This may be extended if paying and filing on Revenue online ROS) In calculating your Preliminary Tax payment you should ensure that it covers your liability to PRSI and Universal Social Charge, as well as Income Tax.
To avoid interest charges, the amount of preliminary tax paid for a tax year must be equal to or exceed the lower of:
90% of your final liability for the tax year, or
100% of your final liability for the previous tax year, or
105% of your final liability for the pre-preceding tax year. (This option is only available where preliminary tax is paid by direct debit and does not apply where the tax payable for the pre-preceding year was nil).
In practice we generally recommend 100% of your previous years liability as it is virtually impossible to estimate your current years tax liability.