More than 43,000 bank customers in the Republic and Northern Ireland are being refunded after failing to take their cash from Bank of Ireland (BoI) ATMs.
About 14,000 customers will receive 1.3m euro in refunds, while 1.7m euro will be returned to 29,000 customers of other banks who used the bank's ATMs.
The average refund is 93 euro including 728 transactions in Northern Ireland totalling £34,721.
There are 17m ATM transactions valued at £1.3bn carried out annually by BoI.
When users complete a transaction and walk away from an ATM machine, leaving their cash and/or card behind, the machine pulls the money or card back in after a certain period of time.
However, during a four-year period ending in October 2009, those users' accounts were still debited for the cash by Bank of Ireland.
Bank of Ireland said the issue arose following the installation of anti-fraud measures, which resulted in the normal system response of an automatic refund failing for these particular transactions.
The bank discovered the issue during internal monitoring.
Bank of Ireland has apologised for the error and said it had enhanced procedures for handling "all such incomplete transactions so that customers accounts will not be debited".
The bank said about six people per ATM machine per year walk away leaving their cash and/or card.