you can do it part way through the tax year they will increase your credits by the amount of tax releif due or you cam make a claim at the end of the tax year however there is a 4 year time limit i.e. you can claim in 2006 for 2005/4/3/2 but not as far back as 2001
you need to complete and sumbit a form called rent 1 available from revenue.ie, revenue will then either process the refund or re-issue your credits.
[broken link removed]
i did exactly that about 3 months ago - claimed for last year and this.
got refunded the full amount for last year plus the first couple of months of this year plus my credits adjusted for the rest of this year. it took all in all about 6 weeks from application to money in the bank. very impressed.