Prisons have been compared to asylums by representatives of the Prison Service at the Public Accounts Committee.
Prison Service Director General Michael Donnellan also told the committee that mental health is a "massive problem in prisons".
He said 30 people are waiting for a place in an acute mental facility at any one time.
Responding to the Independent TD Catherine Connolly, he said 70% of the prison population have an addiction problem, particularly with drugs and alcohol.
Mr Donnellan said the prison service provides an important role for prisoners to detox and come off drugs with medical help.
The committee heard violence is becoming more of a feature in prisons usually because of the unpredictability of drug and mental health issues.
There were 91 assaults on prison officers in 2015, however this was down from 151 in 2014.
Meanwhile, the number of people imprisoned for the non-payment of fines fell by about 1,500 last year compared to 2015.
Mr Donnellan said the 2014 fines act has had a small impact.
He pointed out that people who failed to pay television licence fines or motor fines in the past, were given a €300 fine and 3 months in prison.
Mr Donnellan said people can now opt to pay in installments and he said prison numbers have been relieved.
Given the life cycle of fines imposed are normally between one and three years, Mr Donnellan said he expects numbers to decrease further in the coming years.