Just moved back from Dublin...
Best bet is the Wicklow mountains south of Dublin, pretty sure there are a couple of sites down that way, one is close to Bray/ Enniskerry so about an hours drive from Dublin centre.
I'm pretty sure there is also a site in County Cork (there was a few years ago).
check out:
www.skirmish.ie
www.crossfire.ie
e-mail
paintballwargames@hotmail.com think this site is closest to Galway as it is near to Westport in County Mayo (north of Galway).
My knowledge of the west coast is pretty sketchy but there are plenty of non-paintball things to do to keep you busy around Galway.
I'd recommend getting down to Kerry if you can, Killarney National Park is worth the trip, as is a visit to Dingle.
If you do head to Mayo, try a pint of 'special', Smithwicks with a Guinness head!
Hope this helps