Around 85,000 AIB customers are set to receive a €25 rebate from the bank as a result of a process error.
The mistake relates to people with mainly smaller loans who looked for a top-up on those payments.
The total repayment amounts to more than €2m.
Business Journalist Vincent Wall explains what happened.
"The bank has informed customers this morning that it is to make average repayments of €25 to about 85,000 customers as a result of what it calls a process error linked to top-up loans for personal customers.
"The amount of the repayment in total will be more than €2.1m," he said.