The Larkhill & District Credit Union has said Grant Thornton has been engaged to carry out a review of unauthorised transactions on a small number of accounts.
The credit union in Whitehall in Dublin said the independent accountants' review is still ongoing and the board is unable to make any further comment at this stage.
However, the board stated that the account holders affected by these issues, as well as the Central Bank, gardaí, the Irish League of Credit Unions and insurers have been notified.
Gardaí have confirmed that they are investigating a suspected fraud at a credit union, as reported in the Irish Independent.
In a statement, gardaí said the investigation was focusing on alleged irregularities between 2002 and 2013 at a credit union in north Dublin.
A manager at Larkhill & District Credit Union in Whitehall said the investigation was focusing on alleged unauthorised transactions.
The Central Bank said it was aware of the report and that the issue was not regulatory but one for the gardaí.
The Irish Independent reported that a sum of €400,000 is believed to have been involved.
An Irish League of Credit Unions spokesperson confirmed there was an ongoing investigation into irregular activity in a small number of members accounts at Larkhill & District Credit Union.
It said members that have been affected have been contacted by the credit union and that a process of issuing refunds was ongoing.
The ILCU said: "this is a rare occurrence and Larkhill & District Credit Union is working with the relevant authorities to insure the issue is dealt with comprehensively and brought to a satisfactory conclusion."