MBNA is refunding €18m in interest charges to its credit card customers.
The finance company says it has identified an error in how interest was described to customers in some circumstances.
Hundreds of thousands of account holders may have been affected.
MBNA says it made a mistake in how it described interest rate charges in its terms and conditions.
It says it applied interest as outlined in its marketing material - but a drafting error in its terms and conditions from 2007 meant it applied interest to some accounts that it should not have.
On foot of this mistake, a full review on all terms and conditions dating back to 1997 was carried out - and another mistake was found.
MBNA also failed to outline how interest would be applied on the annual Government Tax and as a result, the company is refunding any interest that may have been applied on accounts up to 2007.
Customers can expect notification from MBNA if they are affected and they will be paid by March next year.
The Financial Regulator has been informed and the company has issued a full apology.