The warnings come from the Prison Officers Association.
The number of people in jail has increased by over 5 per cent over the past year.
There are over 4,000 inmates behind bars - over 200 more than at the same time last year.
The Prison Officers Association says overcrowding could lead to violence or even a murder.
They're raising the issue at their annual conference, which is continuing in Sligo today.
— Irish Prison Service (@IrishPrisons) April 27, 2022
Some of the overcrowding issues have been linked to more court activity after the pandemic.
POA general secretary, John Clinton, says they've seen before that extra prisoners yields negative effects.
"For example in Cloverhill, there's 10 per cent of prisoners on the floor last night. That was 35 to 40 extra prisoners," he said.
"In the Midlands we had 58 sex offenders and only 36 cells."
"We're very concerned with regard to overcrowding."