What kind of landline are you calling? A regular Irish national number or a "special" one - e.g. 1850, 1890 etc., an international number etc? Only regular Irish national landline numbers are covered by membership/add-on landline minutes allowances. For others you need cash credit on your account to pay for them. If calls to regular Irish national landline numbers are failing but you have landline minutes on your account then you'll need to contact 48 support to get them to explain/fix it. See the link in my signature below.
Hope this helps.
I am not a 48 employee so if you need to contact 48 support then use option 3 or the chat facility here.