Prisoners subjected to 22 and 23-hour lock-up are at greater risk of mental health, campaigners have said.
With more than 200 convicts limited to one or two hours a day outside a cell, the Irish Penal Reform Trust (IPRT) has claimed that the practice should only ever be used as a temporary arrangement.
It claimed that prisoners on lock-up for 15 days or more are at risk of irreversible harmful psychological effects from insomnia and confusion to hallucinations and mental illness.
IPRT executive director Liam Herrick said maximum lock-up should be a fortnight at a time.
"The potential harm to prisoners' mental health that can be caused by extended periods of isolation means that the practice of holding any category of prisoner on 22 or 23-hour lock-up must only ever be a temporary measure," he said.
"Meaningful rehabilitation is simply impossible in such conditions and this damaging practice will have long-term negative consequences for the prisoner and for society."
The Irish Prison Service (IPS) said that it is actively considering using the St Patrick's Institution - which is to be closed as a young offenders' centre - as a single unit for prisoners on lock-up.
As of Monday, 150 inmates were on 23-hour lock-up - 114 for their own protection or the protection of others; 32 for discipline reasons; and four for medical care. Another 61 are on 22-hour lock up - 57 for their own protection or the protection of others; one for discipline reasons; and three for medical care.
The IPS said 4,100 people are in custody in the country's prisons and about a third of those on lock-up are not being held in single cells.
"Our primary responsibility is to protect the safety of our prisoners in our care. Prisoners on restricted regime are under review and this could involve transfer to another prison where they might not be required to be on the regime," an IPS spokesman said.