Draff,
I think Brendan's response answers your question, however, to add some comments in hope of being helpful...
If the debts were in joint names of your mother and father then your mother is still liable for the debts but as Rainyday said it is worth checking the Terms and Conditions and if there were any insurance policies to cover such loans.
If your father ever had a mortgage with Ulster Bank then even if the UB debts were in his sole name it would be necessary to check if the bank can rely on the security of the house...what you are looking for in the Terms and conditions is a reference to an "All Sums due clause".
If none of the above apply ... why do you, or your mother, think your mother will have to pay anything?