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 part of its efforts to manage the outbreak.
In a statement, the Prison Service said: "The Irish Prison Service is currently managing an active outbreak of Covid-19 in Cloverhill Prison. To date, a significant number of symptomatic prisoners have undergone testing and are awaiting test results. The Irish Prison Service can confirm 1 (one) prisoner in the B Division has tested positive for Covid-19.
"A Cloverhill Prison Outbreak Control Team (OCT) has been mobilised and is engaging with Prison Management to oversee appropriate actions to mitigate against possible spread of the disease within the prison."
They add that infection control measures "may impact on access to video visits and physical visits" for now, but officials are working to ensure family contact can be facilitated "as far as possible".
There are 407 prisoners in the jail, which is one of the biggest in the country.
