AIB customers with small personal loans are to be repaid 25 euro following a technical glitch.
The error is linked to top up loans, and affects around 85 thousand customers.
There was an issue with the way interest was transferred meaning the rate for these top-ups was higher than it should have been.
The total repayment is expected to cost 2.1 million euro.
Customers have been informed and further details are available from AIB.