The general opinion is any system is only as good as the maps it's based on. Garmin seem to come out top for Ireland from most users.
The actual hardware will have some differences (some give street names, some don't etc.) but having the best maps is definatly the key.
(Don't have one to give a personal suggestion of, but a few previous threads on Sat Nav's on AAM [specific to Ireland] and lots of discussion in general online)