Ireland Trip

Vodafone & O2 had very decent coverage when I was there in 2005. AFAIK all of European carriers offer some sort of a prepaid service, so you should have no problem picking up a SIM. It would help if you already had an unlocked world-capable (GSM 850/900/1800/1900 at least, 3G 850/1900/2100 a plus) phone. You can see coverage maps here.

Note: the bands in red are the ones you need in Ireland.

 

There are many service providers who provide SIM card in corner of the country. You just need to call or mail them. They contact you back. They provide prepaid SIM within 2 or 3 days.