To call UK numbers you need an active membership and cash credit on your account to pay for the international calls.
https://48.ie/charges/international-calls
Northern Ireland landlines dialled with the 048 (rather than 00 44) prefix are covered by your membership/add-on can minutes.
https://community.48.ie/t5/The-Knowledgebase/International-Calls-amp-Texts/ta-p/53116
