Covid-19 has spread to the highest security prison in Ireland, Portlaoise Prison in Laois.
Two prisoners had tested positive for the virus initially, confirmed by the The Irish Prison Service last Saturday, January 9.
On Monday they confirmed that a third prisoner had contracted Covid-19, with all the cases contracted while in the prison.
"Mass testing" of prisoners and staff took place on Sunday and Monday, following the outbreak.
The prison had up to now had no cases among prisoners. The Irish Prison Service said that it cannot comment on whether staff have contracted Covid-19, as this was a matter for the Health Service Executive.
One prisoner in Mountjoy in Dublin has also tested positive.
"The Irish Prison Service can confirm an additional prisoner has tested positive for Covid-19 in Portlaoise Prison. 2 prisoners in Portlaoise Prison and 1 prisoner in the Progression Unit in Mountjoy Prison have tested positive for Covid-19. The prisoners were not recently committed to prison therefore they have contracted the virus within the prison.
"Families of the infected prisoners have been informed.
"Outbreak Control Teams (OCT) have now been established in Portlaoise and Mountjoy to agree and oversee the appropriate actions to be taken to mitigate against further possible spread of the disease within those prisons," the IPS stated.
"The Service is working closely with Public Health in the HSE with regard to the management of the current outbreaks including making arrangements for the mass testing of prisoners and staff in both locations," they said.
Mountjoy prison testing is taking place this Wednesday and Thursday with a further test to be carried out 7 days later.
"To help prevent the spread of infection a more restricted regime has been introduced in both prisons which may impact on access to video visits and phone calls in the short terms for prisoners in precautionary isolation however we are working to ensure that we can continue to facilitate family contact as far as possible."
Portlaoise Prison is Ireland's only high security prison with some 290 inmates, sited alongside the larger medium security Midlands Prison which has a capacity of 870.