Your mortgage provider or the Revenue TRS Section should be able to tell you if form TRS1 was filed for the mortgage. If so, then you are getting the relief, if not then you need to file TRS1 now.
Revenue TRS section have form for back years. Current year usually done with refund via bank. As so close to y/end that might be difficult. All claims subject to the 4 year rule ( 2004 must be claimed by 31/12/08, 2005 by 31/12/09 etc. )