Credit card company MBNA is to refund around €18m after mistakenly overcharging hundreds of thousands of customers.
The company has reported the matter to the Financial Regulator and has apologised.
Any credit card that carries the name MBNA on the back could be affected by the overcharging.
The company's cards are affiliated to EBS, Ryanair, Postbank and AXA.
In some cases interest was mistakenly applied between the date a consumer bought a product and the date it issued a statement to customers.
There was also an error in how it charged for cash withdrawals and for the Government's credit card levy with some of the issues stretching back to 1997.
Any customer affected will be repaid money owed plus interest by March 2010.
The company says the majority of customers are likely to be refunded around €38 or less.
MBNA is Ireland's third largest credit card company.
