Probably the usual iPhone roaming problem due to the stupid way that Apple do things and how it affects the use if iPhones on virtual networks such as 48 (and Tesco etc.).
See here and you may need to try that many times before it works - toggling airplane mode on and then off and trying it quickly may allow the phone to briefly catch the local network allowing the reconfiguration to be carried out.
Hope this helps.
BTW - do all calls and text fail or just calls to Irish landlines?
If so are you sure that you have landline mins left?