Suspected financial irregularities at another credit union have been reported to gardaí and the Central Bank.
Members of Citybus Employees Credit Union in Dublin have been told their savings have not been affected.
But the annual accounts and financial statement for last year, seen by the Irish Independent, describe "financial irregularities" being uncovered, including problems with a member draw which meant losses of €218,000, and a tax liability for directors.
Outgoing chairman David Mooney wrote in the report: "An Garda Síochána has been fully informed."
It comes after financial irregularities have been uncovered at a small number of credit unions.
The Citybus Employees CU annual accounts state that members who entered into the prize draw had had their money paid back to them.
"In mid-2016, our internal systems and controls identified an irregularity in the operation of the members' draw. The accumulated loss suffered by the credit union ran to €218,547," the accounts state.
The credit union has made a claim on its insurance to cover the loss. It is not clear what the issue was with the draw.
An underpayment of tax on the expenses given to directors has prompted the credit union to make a settlement of €61,000 with Revenue.
The annual report calls this a "history tax liability".
It relates to benefit-in-kind tax on Football Association of Ireland tickets.
Asked about the "financial irregularities", Citybus Employees CU manager Louise Kennedy said an issue warranting further investigation was identified through internal processes during a routine review of the prize draw.
"The credit union took all appropriate steps to deal with this issue swiftly and immediately informed the relevant authorities - the Central Bank and An Garda Síochána," she added.
Ms Kennedy insisted that member's savings were not affected. "As this matter is currently under investigation, we are not in a position to make any further comment at present," she said.
The issues are due to be addressed at the credit union's annual general meeting this week. It has 2,800 members.
This is just the latest controversy to hit a credit union. Last year one of the biggest credit unions in the State, St Anthony's & Claddagh in Co Galway, dismissed a staff member and reported a suspected theft to gardaí.
Gurranabraher Credit Union in Co Cork lost €200,000.
Last month, a 55-year-old former employee of Synergy Credit Union in Fermoy, Co Cork, pleaded guilty to stealing more than €400,000 from the lender.
Rush Credit Union, in Co Dublin, was shut down after a series of fraudulent activities were discovered. Some €1.3m has been set aside to cover the cost of the fraud at that lender.
A Central Bank spokesperson said it expected all credit union prize draws to be operated in accordance with legislation, regulation and guidance and be fully transparent to members.
"Credit unions operating prize draws are expected to review the recommendations set out in that report and consider how these can inform their boards in developing or updating the systems and controls in place for such draws," the spokesperson said.
In March, a Central Bank review of credit unions found that in 30pc of those that operated prize draws, staff and directors were winners of prizes. Some credit unions reacted to this by banning staff and directors from entering the draws, which often have a car as the top prize.