I have received the following responses to your question from the covered institutions.
BOI
Bank of Ireland deals with any queries related to matters such as instances of potential overpayment on an individual customer basis. Any potential issue, which affects a greater number of customers, is dealt with through established processes and procedures, in consultation with regulators, where appropriate.
AIB
Any queries related to matters such as instances of potential overcharging are dealt with on a case by case basis by the bank with individual customers. Any potential broader issues which affect a greater population of customers are dealt with via established procedures in consultation with the Central Bank as regulator.
PTSB
Overpayment or overcharging gives rise to the same customer remedy - refund of the amount overpaid or overcharged. Compliance doesn't keep records of complaints - just amounts refunded. Most amounts refunded don't arise from a complaint - one complaint can result in refunds for many similarly affected customers.
IBRC
It is not the practice of IBRC to publicly disclose detailed information in relation to alleged incidents of overpayment by customers. If such an incident was to occur, the Bank would deal directly with the individual customer in an effort to resolve that issue directly through the Bank's formal complaints handling process as required by the Central Bank Ireland Consumer Protection Code (CPP). The CPP includes information for complainants who wish to escalate matters to the Financial Services Ombudsman.