Nearly a fifth of prisoners are now being locked up in their cells for more than 19 hours a day.
A new report from the Irish Penal Reform Trust has raised concerns about the scale of overcrowding in many facilities.
It says that's leading to some inmates having to sleep on mattresses on the floor of their cells.
Executive director Fiona Ni Chinneide believes the lack of capacity in the system is causing significant problems:
"The service will have less flexibility about how it can separate people across the system if it is absolutely packed," she said.
"This in turn places a further burden on the service which has to continue to provide education, training family visits (and) access to medical derives to all of these people who are locked up," Ms Ni Chinneide added.
There's a warning about an increased risk of violence in prisons because so the overcrowding.
The number of people behind bars is at a four-year high at almost 4,000.
There are also lengthy waiting lists for prisoners to get mental health supports.
Director General of the Irish Prisons Service, Caron McCaffrey, says they're trying to address that.