Section 244 of the Taxes Consolidation Act 1997 provides for mortgage interest relief (MIR) in respect of qualifying interest paid in a tax year.
The allowable relief is based on the actual amount of interest paid by the borrower. For example, where the borrower pays the correct mortgage amount in accordance with the terms of the loan then the full MIR entitlement is applied to the loan by the mortgage provider as tax relief at source (and subsequently reclaimed from the Revenue Commissioners).
Where the borrower does not make payments or pays less than the full amount of interest due, then the MIR entitlement is reduced to reflect the actual amount of interest paid. If the borrower subsequently repays the arrears then the appropriate MIR will be paid subject to the ceiling (maximum amount of interest allowable) for the year in which the payment is made.