MORE than 300 prisoners are forced to spend at least 17 hours of their day inside their tiny jail cells, the Irish Prison Service has confirmed.

Authorities said that the majority of the inmates were only being given such restricted hours out of their cells because of risks to their safety.

In total, 317 inmates are subject to the restricted regime.

The largest number of prisoners in this category – a total of 113 male convicts – is in Mountjoy Prison, Dublin.

The remainder of the inmates subject to such extended lock-up hours are in St Patrick's (37), Limerick (34), Cloverhill (33), Castlerea (27), Midlands (27), Wheatfield (25) and Cork (21).

The Irish Prison Service said: "On 15 April 2010 there were 317 prisoners falling into the category... This represents just under 8% of the prisoner population on that date.

"The vast majority of these cases are prisoners who require protection and whose regimes have to be restricted for their own safety. There are also a small number of prisoners who are subject to a restricted regime because of medical reasons."

The Irish Prison Service said measures had been taken to alleviate the problem, including the opening of a new separation unit at Mountjoy.

On a normal day, up to 900 prisoners – or 20% of the prison population – are "on protection" after being threatened. Announcing his resignation as governor of Mountjoy last week, John Lonergan described conditions in the jail as "brutal" and "appalling".

A spokesman said: "By and large, there would not be restrictions on all of [the inmates] because they would be in specialised protection wings with other protection prisoners.

"The remaining prisoners are people who are in the ordinary prison system, who are subject to restriction in the interests of their own safety."

Sinn Féin's Aengus Ó Snodaigh said it was inhumane to have people incarcerated in small cells for such a long period of time each day.

He said: "It is not acceptable that people are confined to cells because of overcrowding. The whole point is to rehabilitate people in prison and this cannot happen when they can't access training or other facilities.

"There is also a human-rights issue. These people have not been convicted to be sent to a virtual solitary confinement and they are entitled to exercise and some freedom of movement."

Prison sources said the problem was a direct result of the chronic overcrowding in the penal system.

There is any number of reasons why inmates might find themselves needing protection, according to prison officers.

Sex offenders are at risk, as are criminals who are believed to have informed. The rising number of inmates joining gangs was also causing enormous difficulty, the officers said.