Hi Shkyler,
You will need to fill out the following pdf form : [broken link removed] to claim mortgage interest relief for preceding tax years. Print this form, fill it out and FREEPOST it to the Collector General's Office, TRS Section, Sarsfield House, Francis Street, Limerick. Please note you can only claim for the four preceding tax years to the current tax year, i.e. 2005, 2004, 2003 & 2002. When you apply online for mortgage interest relief this is only backdated to the 1st January in the tax year in which you applied. So you will already be getting interest relief for the current year. You will need to call Permanent TSB to find out how much mortgage interest you have paid in previous years. No harm in requesting from them a Certificate of Interest for each tax year for your own records as claims for mortgage interest relief is subject to audit. Please note it states that it can take up to 12 weeks to process your claim! No surprise there!