Duff Cycles in Omni Shopping centre (also have one in Donaghmede) done a fine job on a bicycle for me, without mentioning the awful bodge that someone *ahem* had made of it. They do weekend (Saturday/Sunday) work which I found particularly useful.
I've also had great results from Wheelworx in Fonthill Retail Park in Lucan. They accept drop off and pick up on weekends too, and were comprehensive in their fixes.