The event took place in March.
Mazaars will investigate a gathering of 30 senior prison staff in the midlands prison in March.
The event was held in the canteen of the Portlaoise facility to mark the retirement of governor Ethel Gavin.
At the time, the prison officers association expressed concern about its impact on efforts to keep the virus out of the prison system.
The Irish Times reports that since March, the Irish Prison Service has received a protected disclosure about several issues in the midlands prison, prompting the decision.