In Ireland, mortgage interest can generally be claimed as an expense against rental income where the borrowing was used to buy, improve or repair a qualifying rental property. Conditions can include registering the tenancy with the Residential Tenancies Board (RTB), while repayments of capital are not treated as interest for this purpose. Relief reduces taxable rental income rather than providing a guaranteed cash refund, and its effect depends on your circumstances and records. Check current Revenue guidance or speak to a qualified tax professional, especially if the property is vacant or the loan has mixed personal and rental use.