You qualify for relief on a maximum of €10K p.a. If you pay more than this then you get relief on €10K. If you pay less then you get relief on the amount under €10K actually paid. Let's assume that you pay €10K+ interest. Then you get relief on the €10K cap at 20% so that's €10K @ 20% = €4K p.a. or €4K / 12 months = €333.33 p.m. This is the amount by which your normal repayments will be reduced by the lender once your application to Revenue for owner occupier mortgage interest tax relief has been processed.