There are two issues here you should keep separate.
1) Do you actually owe the amount the agency claims you owe or are they trying to defraud you? Their refusal to give you a breakdown of the money they claim you owe seems fishy but the only way to be sure is to contact AIB. You are entitled to a precise breakdown of the money you allegedly owe.
2) Assuming you genuinely do owe the amount claimed, should you pay? This depends on (a) how much your credit rating is already damaged, (b) on how much further it would be damaged by not paying and (c) on how much this actually matters to you (e.g are you planning to take out a loan or a mortgage?).
If I was you I would proceed by obtaining clarity on the first issue and then, if relevant, taking professional advice on the second question. MABS should be able to help you with questions surrounding credit rating.