The number of people sent to prison has fallen by nearly a third this year.
Nearly 1,200 more people had been jailed at the same time in 2019, according to the Irish Prison Service.
The service found that 4,173 people were sent to 13 prisons across the State in the first six months of last year.
That dropped to 2,997 in the same period in 2020.
Courts sittings were scaled back during the pandemic and there was a drop in crime.
Fíona Ní Chinnéide, from the Irish Penal Reform Trust, fears prison populations will rise sharply now courts are returning to normal.
"At the outset of the pandemic, Irish Prisons were way overcrowded, you had people sleeping on mattresses on the floor.
"I mean, in the best of times overcrowded prisons do not support rehabilitation and lead to increased tensions, drugs and violence, but Covid-19 brings an additional layer to this.
"A return to prison overcrowding will put the lives of prisoners and prison staff at risk.
Separate statistics show that, on June 30, there were 101 Polish people in jail in Ireland - the highest group of non-nationals.
The next highest were 79 Romanians and 77 Lithuanians.
The Irish Prison Service has put the drop in new inmates down to the pandemic.
However, Ms Ní Chinnéide says the number of people in prison is still too high.
She said: "To achieve social distancing in a way and manner that doesn't impact on the longer physical and mental health of the people in prison, you must get the numbers down to, ideally around 3,000 or 3,100.
"Recently in June it was down to around 3,650 and now we are seeing it on the rise again which is a big concern."