Your membership is required but will not cover calls/texts while abroad (and NI is considered "abroad" in this context).
You will need cash credit to cover the cost of outgoing and incoming calls and outgoing texts or else EU roaming calls and/or text add-ons.
http://www.48months.ie/membership-plans/going-abroad
Data roaming will not work abroad (including NI) so you will have to use wifi where available to access the internet.
Hope this helps.
