There are 407 prisoners in the jail, which is one of the biggest in the country.
About 40 inmates in Cloverhill Prison in Dublin are being tested for Covid-19, as they have symptoms of the virus.
So far one prisoner has tested positive, and the rest are waiting for their results.
The Irish Prison Service says all staff and inmates will now be tested, as it manages the outbreak.
There are 407 prisoners in the jail, which is one of the biggest in the country.

AI Chatbot Restrictions Bill For Under 16s Before The Dáil
Ireland Not Prepared For Extreme Climate Emergencies, Warns CCAC
Clane Student Remanded On Bail In Relation To Bomb Interception In Monaghan
Two People Killed In Offaly Workplace Incident
Further Fuel Protests Not Ruled Out, Says Castledermot Agri Contractor
Commission Of Investigation To Be Carried Out Into CHI
Orange Order Parade Delayed As Protesters Block Garvaghy Road
Controversial Orange Order Parade For Nationalist Road Gets Go-Ahead