AIB is to refund €8.7m to 76,000 ATM customers across Ireland.
The refund is being made to customers who withdrew money from the machines but did not take the cash when it was dispensed.
The money was left behind at AIB ATMs between 2003 and 2010.
AIB began an investigation last July after Bank of Ireland announced that it was refunding €3m to 43,000 customers who had made withdrawals but forgot the money.
AIB will refund €5.1m to 41,000 of its own customers and €3.2m to 30,000 non-AIB customers.
A further £350,000 (€400,000) will be refunded to 5,800 First Trust customers in Northern Ireland.
AIB estimates that the cash is left behind in approximately 1 in 3,000 ATM transactions.
There is a 30-second window to take the cash at an ATM. AIB says that at some busy machines three people a day leave their money behind them.
AIB says anti-fraud measures prevented them refunding the money before now as a previous policy required customers to contact the bank themselves if they did not take their cash from a machine. The bank could not contact the customer.
AIB says refunds will be lodged into its customers' accounts by midnight tonight. First Trust customers will receive refunds next week.
It may take longer for refunds to reach non-AIB customers as AIB must write to each of the banks of the affected customers directly to ask them to process the refunds.
AIB apologised for the error and said that process and procedure changes have been put in place to prevent this happening again.
AIB to sell stake in Bulgarian bank
AIB is selling its 49.99% shareholding in Bulgarian-American Credit Bank.
This is a publicly-listed bank based in Sofia, Bulgaria, focused on small to medium sized enterprises.
AIB has not revealed what it is selling its shareholding for, and the deal has to be passed by Bulgarian regulators