Phones bought in USA work in Irl?

Can anyone tell me if, in general, mobile phones bought in the USA will work in Ireland?
I'm considering buying one but don't know if it would be compatible with a Vodafone in Ireland account.
I don't have a specific one in mind and haven't researched what's available and how prices compare etc until I find out if it would actually work.
I'm not up to date on the whole mobile phone thing so any information on this would be great.

A friend of mine from canada come over to visit once and he left he phone charger behind. He was able to put his sim card into my phone and it worked. My phone had been unlocked so thats why it worked. I'm sure it would be the same anywhere else though, as long as you get the phone unlocked you can put any sim card intot he phone.

My cousin from the US visits quite a lot, and her phone doesn't work here, although we have never tried it with an Irish SIM card. You could email the seller and ask.

I bought a TracFone in LA a few years ago. Use it with USA sim when on holidays, but it can't be unlocked here. At least not by the shops I tried. As this is a cheapo it may not be representative.

We bought a pay-as-you-go Motorola Razr from T-Mobile in the US, got it unlocked using an online service and can now use with any sim card - have used it in 6 different countries so far.

i think its something to do with the band the phone operates on?

you know like some phones are tri-band....means they can pick up a signal most places, ie. my old nokia N95 8GB worked in new york last year.

http://en.wikipedia.org/wiki/Tri-band