Hundreds of prisoners are being forced to sleep on the floor of jails because of accomodation shortages.
New figures from the Irish Times show 572 people - 404 men and 168 women - had to sleep on mattresses on the floor at Limerick prison in July.
The number was 674 at Midlands Prison in Portlaoise and 119 in Cork.
The Irish Penal Reform Trust says the scale of the problem is "unacceptable" and a clear breach of human rights.
The monthly total of prisoners sleeping on floors in Castlerea Prison in July was 115 - an average of four per night.
Deirdre Malone, executive director of the Irish Penal Reform Trust, said the scale of the problem is “entirely unacceptable, and in clear contravention of basic human rights norms, including the European Prison Rules”.
She said this should not be happening in 21st century Ireland.
Ms Malone said overcrowding decreases the chance of meaningful rehabilitation, increases tensions and makes prisons less safe for prisoners and staff.