Last year I claimed three years of past Rent Relief for Private Rented Accommodation. I got a nice refund from revenue for the past 3 years but the amount was less than I expected.
On my Tax Credit Certificate for 2008 the Rent Tax Credit is €400.
On Revenue website they state [broken link removed]
Relief is due at the standard rate of tax (20%) in the tax years 2007 and 2008 subject to the following upper limits:
Single Under 55 max. €2,000
If you are under 55 years of age, it is 20% of the rent you paid for the year, or 20% of 2,000, whichever is the lower. In your case 20% of 2,000 is the lower figure, so that's what you got back. The 2,000 is the ceiling of allowable rent for taxation purposes.