I reciently purchased a used car in Dublin and can highly recomend Ultimate Cars, Killeen Road, Dublin 10, 086 258 0981. They gave me a 12 month guarantee and have stood behind it, fixing a couple of minor problems. My car is a 9 year old Honda Jazz so the guarantee is a real plus.
Eugene Hogan at Ultimate Cars also agreed to buy the car back when I am ready to leave as long as I don't crash it and keep it in good condition.
I will only be in Ireland for a year but buying seamed better than renting although the insurance is high. I ended up getting insurance with my USA drivers license through AXA which was the only company I could find to insure me w/o a Irish license.