When calling Northern Ireland landlines you can use the normal UK +44 prefix or you can use the 048 prefix described here:
A call to NI using +44 is charged as an international call at 50c per minute.
As I mentioned here a few times I was never compeltely sure how calls to NI using 048 were charged.
Well 48 support just clarified to me that calls to NI using 048 are covered by your membership or add-on bundled landline calls allowance.
So if you're calling an NI landline it makes sense to use 048 rather than +44.
Refer to the wikipedia link above for instructions on how to use the 048 prefix (basically you replace the 028 prefix in an NI landline with 048).
Hope this helps.