I've done some research for a trip to Italy my wife and I are planning...having communications there is one of the reasons I chose to get the Razr recently. I think there are a couple options depending on how much voice, text and data you'll be doing. The easiest and surest is the Verizon International Roaming plan as has been suggested. That can add up though if there is a fair amount of activity as the rates are not the best. Second, best, in my opinion, based on better rates and convenience would be to get one of the pre-paid international SIMs before you go. I've looked at a couple companies that offer this:
www.telestial.com and
www.onesimcard.com. Both of these claim good reviews from some prominent tech magazines and the like and both offer voice and text rates better than Verizons. Data rates look to be higher though. I've not uses these yet but they seem to be attractive. Each of these also offers simple international phones which may also be an option. Third, get a local SIM when you get there, as you mentioned. The Razr, as I understand, is unlocked for International SIMs so this may work well too and be the least expensive. Not so high on my convenience scale as you have to find and go somewhere to get the thing. On the other hand, in an English speaking country, it may be less troublesome to convey what you want to do to the techs there and, as I understand, the pre-paid SIMs are pretty easy to find there. This may also be your best bet for data rates. Verizon looks to be $.25/MB ($25 per 100MB). It may be wise to research this of you are going to use data. Option 3 may be the better choice!
EDIT: Did a quick google for Ireland SIMs specifically, it looks like there are much cheaper options out there. Especially for data. A google of "sim cards ireland uk" pops up some options that look good...at least to my inexperienced eye!
EDIT 2: On the other hand, options 2 and 3 may mean that you will have a UK phone number which would mean international rates to whoever is calling and texting. I have had a tendency to forget about that. Option 2, though, has a US number or has the ability to add a US number.