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.
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).