The Central Bank of Ireland says it held the personal credit histories of thousands of people for longer than allowed, potentially making it harder for borrowers to get loans.
The Bank says it recently identified an "archiving error" that affected the retention period of certain borrower information held on the Central Credit Register.
The data was held on the register for three months longer than the limit of five years because of a technical error that affected the automated deletion process of historical records in the CCR, says a statement.
This meant that around 20,500 borrowers had data pointing to repayment difficulties during the three month period that should not have been on the register.
The error constitutes a data breach under data protection legislation.
"At this stage...the Central Bank is not in a position to determine with accuracy the extent to which credit applications were adversely affected by this incident," says the statement.
Adds the bank: "We are engaging with lenders to advise them of the incident and to establish if the additional three months of historical credit information affected their lending decisions in any way."