Eurocycles (across from the cathedral). They're brilliant, don't know if they do new bikes but they have a load of second-hand ones. Any problem I've ever had with my bike they've fixed it up for half the price other shops charge. They're also honest and only fix what you ask them to (i.e. that wobble in the front wheel which you never noticed but it's going to cost you 20 euro to re-align). Only thing is that the owner can be a bit abrupt, so you need to take him with a pinch of salt.