If you're in Dublin, Waterford or Galway first option would be NTL. NTL and ESB have recently announced an agreement which provide NTL's cable modem service in Waterford and Galway. There is another cable modem provider in Waterford around Dungarvan I think. They're offering 8Mbps which is probably the fastest going in Ireland.
I'm fortunate enough to be in a recently NTL-enabled broadband area and it's great. No grief, no problems. 1.5Mbps free for three months and by that time I've been told they'll have upgraded their current packages (300kbps, 750kbps and 1.5Mbps). Far cheaper than their DSL counterparts. ClubMan's ref to the boards.ie is definitely worth a daily visit. You'll get an idea for who's good and who's bad.
Let us know where you are and some AAMbots might pipe up about what they're using.