The President of the Prison Officers' Association, Jim Mitchell, has said prisons are overcrowded to the point where they cannot function as places of rehabilitation.
Speaking on RTÉ Radio's Morning Ireland, Mr Mitchell said some prisons hold almost twice as many prisoners as they should.
The POA is currently holding its annual conference in Kilkenny.
The prison service has said the situation is being dealt with through an ongoing programme of expansion and modernisation.
It said that approximately 1,200 additional prison spaces have been provided.
