This relates solely to Mortgage Interest (rental interest) arising from a rental property.
For Rental Income purposes you can deduct the Mortgage Interest that you have paid in that year.
How much can be claimed?
Since 2009, only 75% of the Mortgage Interest paid in that year can be deducted.
Also, since 2006 one can only claim this deduction if the landlord has complied with the requirement to register residential tenancies with the Private Residential Tenancies Board.
A certificate of interest paid (obtained from the mortgage provider) would be required and must be retained for a period of 6 years, as per Revenue.