The Director of the Central Mental Hospital has said the case of a brain-damaged homeless man in prison is "far from rare".
Professor Harry Kennedy said there is a caseload of around 250 people in the prisons with severe mental illnesses or disorders.
Yesterday, the President of the High Court, Mr Justice Peter Kelly, said it was "staggering" that a homeless brain-damaged man, who is being held on remand in Mountjoy Prison, has been in the prison's high dependency unit for a year, despite persistent reports that he was of unsound mind and needed residential care.
Speaking to RTÉ's Today with Sean O’Rourke, Prof Kennedy said: "That is about 6% of everyone in prison at the moment, 3% of those who come through the doors have those needs.That is about 300 people a year."
He said there is a need for joined-up thinking is the issue.
"Prisons are not hospitals. We do as much as we possibly can. Prisons are very, very caring compared to how I remember when I started doing this work 30 years ago, but the lack of joined-up thinking is a problem."
Meanwhile, the Irish Penal Reform Trust has described the man's case as a "social policy failure" and said adequate care should be provided to him.
It also called for an end to prisons being used to "warehouse the effects of social policy failures".
He was described as having filthy feet and a rare nail disease not seen in Ireland in decades.
His bed linen in the high dependency unit of Mountjoy Prison, was filthy and had not been changed in months, the court heard.
In a statement, Minister for Justice Charlie Flanagan said: "When I became aware of this disturbing case I sought an urgent report from the Prison Service."
He said he awaits that report and said he also spoken with the Governor of Mountjoy.
Mr Flanagan added: "I intend to discuss the case as a matter of urgency with my cabinet colleague Simon Harris.
"I am constrained in what I can say further as the matter is subject to court proceedings."
The Health Service Executive said it would be inappropriate to comment at this time as the matter was still before the courts.
On RTÉ's Morning Ireland, Fíona Ní Chinnéide of the Irish Penal Reform Trust, said too many people were being held in prisons who should be in therapeutic and psychiatric settings.
She said while they do not know the prevalence of people with mental health issues and disabilities in the prison system, the most recent detailed analysis found 16% of men on remand and 27% of men under sentence had mental health issues or mental illness.
For women, she said it was higher, with 41% of women on remand with mental health issues or illness.
Ms Ní Chinnéide said: "Prison is being used to warehouse the effects of social policy failures and that has to stop."
She said while she welcomes the Minister for Justice's statement that he intends to discuss this particular case urgently with the Minister for Health, she said a lack of joined-up thinking across Government departments was the root of the problem.
She said a high level taskforce across the Department of Health, Department of Justice, the HSE, the courts, An Garda Síochána and other agencies was needed and that it should be led by the Department of An Taoiseach.