Prisoners who end up in padded cells are there due to impulsive or chaotic behaviour, not because they are ruthless offenders, according to a report by the Irish Penal Reform Trust.
The reasons recorded by prison officers for using the cells include "aggressive, unstable, depression, medical observation, certified insane and own request."
The report says prison governors believe the majority of prisoners who request a padded cell are suffering from various forms of depression and believe many of their mentally-ill inmates should not be in prison in the first place.
The study details 224 entries to the cells between November 1999 and March this year. It found that 86 entries, or nearly 40 per cent, had either missing entry or exit dates, which made it impossible to determine the length of time the person spent in the cell.
The trust "believes therefore that the findings are a very conservative estimate of how long prisoners spend in solitary confinement," the report states.
It says some mentally-ill prisoners are repeatedly put into padded cells. The longest of the repeat entries recorded was 25 days out of 30. The report calls on the Government to:
refurbish 40 rooms in the Central Mental Hospital in Dundrum which have been empty for almost 20 years to cater for serious and long-term mentally-ill inmates;
set up in-service psychiatric clinics in Mountjoy, Cork and Castlerea prisons with at least six beds for short-term or less seriously ill prisoners;
appoint a full-time inspector of prisons and an ombudsman for prisoners to achieve transparency and accountability;
give consideration to the idea of mental health courts to reduce needless incarceration.
ratify the United Nations Convention Against Torture, Degrading and Inhumane Treatment.
The trust's chairwoman, Dr Valerie Bresnihan, said it had raised its concerns with the Human Rights Commission and was "seriously contemplating" taking a court challenge to the Government.
A spokesman for the Department of Justice said the report would be referred to a group established to review the structure and organisation of prison health services.
The Prison Officers' Association said the mental and medical health of prisoners must be served by community-based services and pointed out that prison staff are not trained or aware of any type of mental illness.
It said a study of prisoners was needed to establish the levels of mental or psychiatric illness and behavioural problems, so that prison officers could be equipped to respond.