Preliminary Tax is your estimate of your Income Tax payable for the year and must be paid by 31 October. In calculating your Preliminary Tax you should remember that it includes PRSI and Health Contribution as well as Income Tax.
The amount of Preliminary Tax you must pay to avoid interest charges is the lower of:
90% of your final liability for the current tax year
or
100% of your final liability to tax for the immediately previous year
or
105% of your final liability for the year preceding the immediately
previous year. This option is only available where you authorise the
Collector-General to collect tax by direct debit. The 105% rule does
not apply where the tax payable for the pre-preceding year is NIL.
See [broken link removed] for a more detailed explanation of self assessment
If the letter came from your accountant then I'd say that you're right, it's their estimate of what they think you owe, and they are looking for you to pay it before the deadline (Oct 31st for post - Nov 15th for online returns).