A memory stick containing the unencrypted personal information of 894 customers has been lost by a Bank of Ireland employee.
The personal information, including account numbers, phone numbers and addresses, was copied onto the USB device in contravention of the bank's policies and procedures.
Bank of Ireland says no financial information relating to customer accounts was on the stick. The bank says it has no reason to believe the stick has fallen into "the wrong hands" but it is taking "appropriate steps to safeguard its customers' interests".
Most people affected by the loss have been informed and Bank of Ireland has launched an investigation. The Data Protection Commissioner is also investigating the blunder.
In April Bank of Ireland revealed that four laptops containing the unencrypted personal details of 10,000 customers had been stolen the previous year.
Less than a month later the bank was forced to investigate claims that another computer containing confidential information had been stolen back in 2001.