3 has a home-from-home agreement and I think o2 (uk) has brought it in too - check online in their international section.
But roaming on 3... half the time I nip over to the UK my 3 phone keeps bouncing around between 3.. T-Mobile.. Vodafone.. 3 again.. o2.. and try to lock it down to 3 (manual selection) it says half the time it's not available. Bit of a joke to be honest. But when it works it's grand.
3 in Ireland won't have the best coverage if you're looking for 'back of beyond' type places. Better stick to o2 or Voda imo.
I'm with 3 and have no problem in back of beyond places.
My mother's with Meteor and that won't even work in Blarney.
My only gripe with 3 is they have no webtext.