You might want to look at the EU calls and text add-ons as they are often more cost effective than paying for services from your credit balance.
Going abroad
Especially if you are going to make some regular calls to Irish numbers and receive calls while roaming - because with the EU calls add-on you can receive calls for free rather than paying by the minute. If you don't use these or they don't cover all of your usage then you will need some cash credit on your account to pay for services.
Other call charges
48 don't support data roaming abroad so if you need internet/data access (e.g. for FaceTime, internet calls) then you will have to be in a wifi area.
Based on some posts here it seems to me that iPhones can be picky when switching networks and roaming so be aware of that being a possibility when you travel. E.g. there are a few recent reports from people roaming who lost services totally when roaming abroad. However it's not clear why they changed their settings or whether or not their phones were jailbroken. So maybe this just works for most people...? It would be good if somebody with first hand knowledge/experience could comment. Certainly roaming abroad (in the EU - UK/NI, France) has worked fine for me on Android.
Hope this helps.
