More than 8 million euro is being refunded to bank customers who made ATM withdrawals without taking any cash.
Allied Irish Banks (AIB) has announced 71,000 people will be paid back the money owed from a period covering more than seven years, with compensatory interest.
It revealed some customers were debited for some ATM cash withdrawals between 2003 and 2010, even though they did not take the cash which was dispensed by the ATM.
An AIB spokesperson said an undisclosed donation will also be made to charity in lieu of several unidentified customers whose accounts were debited as far back as 1995.
The bank said the problem arose as a result of the introduction of anti-fraud measures which prevented the automatic processing of refunds for these particular transactions.
"Process and procedure changes have now been put in place to prevent a reoccurrence," it added.
Some 5.1 million euro will be refunded to 41,000 of its own account holders, with 3.2 million euro paid to 30,000 non-AIB customers.
AIB has apologised and is writing to its cardholders who have been impacted by this problem to inform them of the amount of their refund.
Other banks are also arranging for the AIB refunds to be credited to their customers.
There were a total of 57 million transactions from AIB's 783 ATM machines nationwide in 2010, totalling 7.7 billion euro.