A number of AIB customers have been left out of pocket after their mortgage interest relief was not paid in February.
AIB said the problem is due to an agreement it has with the Revenue Commissioners for paying tax relief at source.
It said the issue affected customers who pay their mortgage on the last day of the month.
In February that fell on a Saturday and because the next working day was in March that tax relief could not be paid.
It is understood the bank has told customers it will pay the money back to them gradually over the course of the year.
But it said that anyone in difficulty because of this issue can contact their local branch for support.
The Revenue Commissioners said it has not received any explanation from AIB about why this happened.
It said it received a number of queries about it from AIB customers but that it is a matter for the bank and not Revenue.
