Some 17 mentally ill prisoners are on a waiting list for transfer to the Central Mental Hospital, which is a "wholly inadequate" and "potentially dangerous" situation for all inmates.

The prevalence of mental illness among the prison population is significantly higher than among the general population, according to the Irish Prison Service.

"All possible steps are taken by the prison authorities in consultation with the treating psychiatrists to ensure that all necessary and appropriate treatment is provided as early as possible," according to a prison service statement.

Psychiatrists from the Central Mental Hospital provide an in-reach service to prisons in the eastern region, consisting of 21 consultant-led weekly psychiatry sessions.

Liam Herrick, of the Irish Penal Reform Trust, described the situation as a "very acute problem", and said that no waiting list should exist when it is recommended that a mentally ill prisoner should be hospitalised. "It's a very serious issue. When kept in prison, mentally ill persons' symptoms worsen. It places themselves and others around them at great risk," he added.

Decisions regarding admissions to the Central Mental Hospital are made by treating psychiatrists based on clinical criteria, according to the prison service.

"While the in-reach service by the CMH for these prisoners is welcome, it is not a substitute for hospital care," said Herrick.

"We acknowledge that the prison service is in a difficult position. The government has been aware of this problem for many years and the HSE has yet to provide the adequate facilities for all mentally ill patients."

In the coming months, the High Court will hear a case taken on behalf of two former prisoners by the penal reform trust.

They are seeking compensation because they claim their constitutional rights were violated in the 1990s were they were kept in prison when hospitalisation was recommended because of their psychiatric problems.

Meanwhile, the murder trial is due to begin this week of a prisoner killed in Dublin's Mountjoy Prison in 2006.

Gary Douch (21) was strangled and beaten to death in a holding cell in the presence of a number of other prisoners. Stephen Egan (23), of Belcamp Crescent, Coolock, Dublin, has been charged with his murder.

Douch was killed in one of Mountjoy's so-called "protection cells". Prison staff found his body smeared with excrement. The other prisoners in the cell had not raised the alarm, fearing for their own safety.