Caron McCaffrey also admitted that overcrowding was a serious problem, but added there has been a notable increase in the imprisonment of people guilty of coercive control and sexual offences, RTÉ reports.

Ms McCaffrey was speaking at the Prison Officers Association Annual Conference in Galway.

A significant number of assaults in prisons are carried out by a small number of mentally ill people, the Prison Service said.

Recently, five staff were injured in an attack by a person suffering from mental health problems.

Each day, 250 people receive treatment from the Forensic Mental Health Service in prison. About 50 per cent are engaging the services of or waiting for a psychologist, while 60 per cent of those imprisoned have an emotional disorder.

Seventeen men and one woman are at present waiting in prison to be admitted to the Central Mental Hospital. Four are so seriously unwell that they have to be “barrier handled”, meaning only prison officers in full riot gear and shields can deal with them.

Ms McCaffrey said the mentally ill are put in penal custody because it is the safest place to bring them, but that they should really be in hospital.

She also said there is a pilot project in Limerick which sees mental health specialists working in garda stations to help identify and treat mentally ill people in the criminal justice system.