Ulster Bank is refunding up to 18,000 customers who were overcharged interest on their accounts.
The bank said it was "an interest overcharging error on our part."
It's writing to business loan and overdraft customers affected.
In March 2012, the bank changed some components of the interest rate charged on accounts.
But it's since found that some customers' accounts did not allow for this change, and they were therefore overcharged.
The bank said the overcharge is, on average, less than one-third of 1% per year - with the average refund being less than €2,000.
Customers are not required to take any action.
But those who have queries can contact a dedicated customer helpline online, or call 1800-719-874.
