Roaming is explained here.
https://www.48months.ie/membership-plans/going-abroadThere are no roaming add-ons.
If you're roaming in the EU then you need a Guido membership and the membership bundled allowances or normal add-ons cover usage while abroad.
EU roaming will not work on any other membership.
If you're roaming in North America or Australia/New Zealand then any membership will do but you need cash credit to cover outgoing calls and texts and incoming calls and data will not work.
Hope this helps.