A total of 71 positive cases of Covid-19 have been identified in three prisons where outbreaks have occurred, according to the Irish Prison Service.
Fergal Black, Director of Rehabilitation and Care, said there were also 49 positive cases among staff across the 12 prisons.
Speaking to RTÉ's This Week, he said: "Prisons are a microcosm of the community. Our staff live in the community and work in the prison. Unfortunately, we’ve had situations where staff who are asymptomatic come into the prison, and as a result of mass testing we've identified that they were Covid positive."
These figures are a big jump for the prison system, which up to last month had identified just 504 cases among staff and 276 among prisoners since contact tracing began in 2020.
"The overall number of prisoners who were identified as positive increased by 44% from last month to this month," said Mr Black, explaining that this was a cumulative figure.
Cloverhill prison in Dublin is the worst affected. As of this weekend, 37 positive cases have been detected: 21 among staff and 16 among prisoners.
A second round of mass testing is now taking place among Cloverhill’s 350-strong prison population. The prison was forced to restrict movements and activities but Mr Black said these had now been relaxed for prisoners with negative tests.
"On receipt of a negative test prisoners were given access to a limited regime - showers, yards, etc."
Mr Black also confirmed that a larger outbreak in the Midlands Prison in Portlaoise had been brought under control. While 47 positive cases were identified, and sections of the prison were initially locked down, he said the prison had returned to normal.
Mountjoy Prison is dealing with a smaller Covid-19 outbreak.
Physical visits remain in place at all prisons, with robust control measures.
"Prisoners must wear masks. The visits cannot have physical contact. There are screens between the prisoner and their loved ones. It's not ideal, but the primary purpose is to maintain the safety of prison populations," he said.
Mr Black said vaccine uptake among staff and prisoners at 10 of the 12 prisons has averaged 85%. The two exceptions, he said, were Castlerea and Cloverhill prisons.
"Notwithstanding the efforts of our clinical staff, peer educators and others, we have witnessed a difficulty with certain cohorts within those prisons. It remains a challenge, but overall, the Irish Prison Service vaccine take-up would compare exceptionally well with our colleagues in other jurisdictions where you have persons in custody," he said.
Asked about the welfare of older prisoners, Mr Black said that they were being provided with a higher grade of PPE and had received advice on how to keep themselves safe. He said the Irish Red Cross had trained over 1,000 prisoners to act as peer educators. These, he said, were playing an essential role during the pandemic.