i think it depends on your situation (ie single or joint) and how long the period is for
as rates and bands changed over recent years depending on FTB or not
2005 and 2006 is capped at 20% of €4,000
2007, 08, 09 is increased to 20% of €8,000
so if your interest was less that the cap in each of these years, you are entitled to 20% back. if it was more, then you only get 20% back of the max.
why have you waited?
you will loose out as you are paying interest on the amount outstanding, so if trs was being deducted, then the OS amount would be less and interest would be lower!!