Yes Kmpell is correct. Tax-relief at source is much better system , particularly for self-employed as you dont have to wait till the end of the year or the following October to get the tax benefits.
Lenders differ in how they administer the relief, but all with the same net effect. E.g. assume mortgage payment €1,000, and TRS of €100.
Some lenders take a direct debit of €900 form the borrower, and collect vthe €100 form the Govt.
Other lenders collect €1,000 from the borrower and refund the €100. It all happens each month within the same month.
If there is any delay in processing the TRS claim, and say you miss the first couple of months of tax-relief, the money comes back to you (or your mortgage account) in a small lump sum once the tax -office gets it sorted.
TRS adjusts fairly promptly to interest rate changes. Allin all in it is a way better system than the older system.