Does this help at all?
https://community.48.ie/t5/The-Knowledgebase/The-Roaming-Troubleshooting-Guide/ta-p/52179
However, many people have been having problems getting things working while roaming abroad, including in NI, and some couldn't get it working at all.
Note also this:
If none of the above helps then you'll have to contact 48 support.
See the link in my signature below or email them at support@48months.zendesk.com
But I'm not sure that they'll be able to do much more to be honest as many people have been having problems with 48 roaming lately and contacting 48 support made no difference... 
