The bank has "contacted all impacted customers to explain the matter and to apologise unreservedly"
AIB has lost confidential information belonging to a number of its customers in the west of Ireland.
The bank has confirmed that information relating to banking facilities was mislaid on Thursday 31st August in Galway.
RTÉ Raidió na Gaeltachta reported that up to 550 customers may have been affected in Salthill and An Spidéal.
In letters sent to affected customers, AIB said the information included names and details of balances.
However, the mislaid information did not include address or other contact details.
Customers' accounts are said to be 'protected as usual'.
In a statement, the bank said: "AIB has contacted all impacted customers to explain the matter and to apologise unreservedly.
"AIB takes its Data Protection obligations very seriously and has reported this incident to the Office of the Data Protection Commissioner.”