If I buy an iPhone 4 in America, will it work in Ireland?

I am travelling to the U.S. next week. I would like an iPhone 4 but I am not due an upgrade from o2 until December.

If I buy one in a shop in America, will it work here on my Irish o2 sim card? I will be buying it sim free over there so I might have to 'jailbreak' it or something I have been told (whatever that means).

Thanks.

look for the AT&T version, the Verizon version is CDMA only.

You have to jailbreak your iPhone in order to work with O2, with all the hassle involved. In US the iPhone are still locked with the telecom operator, you will just purchase a contract free iPhone.
As far as I know, the only country where the iPhone are actually SIM free is Italy.

Maybe it is too late, but now Apple is selling the iPhone SIM free in US:

http://store.apple.com/us/product/MC603LL/A?mco=MjI4NTM2NTM

As indicated, you can buy a sim free version from Apple Stores and that'd be your only option.