If you have the Mobile Network turned off and wifi turned on, yes you should be able to use internet (assuming you can connect to a wifi spot), and keep charges off your bill. Since you have a Tmobile 8320, you might be also able to take advantage of uma to make phone calls. While I haven't tried this myself, I've read on the forums that calls made over uma/UMA to US phone numbers count as any regular call (anytime minutes may be deducted, but no roaming charges). However, this cannot be used to call a local number in Ireland.
If you do get a chance to try that, let us know. Be nice to have additional confirmation about using uma internationally.