Financial services group MBNA will repay 4,000 customers an average of €215 each after they were overcharged when they redeemed personal loans early.
A letter was sent to affected customers this week explaining they had been overcharged.
The errors date back up to two years.
An MBNA spokesperson said the company spotted the issue following a routine internal review.
The issue only affects people who redeemed fixed term personal loans early over the past two years. The total amount involved is €850,000.
The issue does not affect credit card customers.
While customers are not being additionally compensated for the inconvenience, they are being repaid a sum which includes interest at the base rate of the European Central Bank.
MBNA says the money should be transferred into customer accounts within the next three weeks.
Customers who think they might be affected are asked to call free phone 1800 242 661 during office hours.
