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)
Anything with Navteq maps will be fine for Ireland. They finally completed 'full'(ish bearing in my our on-going infrastructure work) earlier this year.
hubby bought me a Tom tom One, had to pay an extra €47 for the irish maps but feel it was well worth the cash. Heard good things about Garmin too - what swayed us towards tom tom was the free updates.