If you have a Guido membership then everything should just work automatically. Calls, texts and internet. If you have any other membership nothing will work. But in that case contact 48 support and they should be able to let you start a new Guido membership "early" in time for your trip. If you have a Guido membership and find that calls and texts work but internet does not when you're abroad then check your internet settings and if the APN field has changed from 48months.liffeytelecom.com then edit it, change it back to that, reboot and see if data starts working. If it doesn't then you'll have to contact 48 support for more assistance. See the link in my signature below. Unfortunately some users find that data never works while roaming in the EU on a Guido membership even though it should. Hopefully that won't affect you.
Hope this helps.
I am not a 48 employee so if you need to contact 48 support then use option 3 or the chat facility here.
"Unfortunately some users find that data never works while roaming in the EU on a Guido membership even though it should." That's been my experience on the last two trips to France/Spain. The online help guys did their best but it just didn't work. Calls and SMS fine, but no internet (unless in a wifi zone, obviously).