The Irish Prison Service is being urged to abolish the practice of solitary confinement.
A new report from the Irish Penal Reform Trust says nearly 400 prisoners are being locked up for at least 19 hours a day.
It also found the majority of prisoners on such 'restricted regimes' are there for self protection.
Executive Director of the Trust, Deirdre Malone, says tackling violence in our jails should reduce the numbers in isolation.
She observed: "There needs now to be a concerted effort [...] to reduce incidents of violence within the system, and to do what we can to support staff and prisoners, and to make prisons safer.
"One of the things we're suggesting is that the Prison Service do a really deep examination of the reasons why there are so many requests... and also examining whether violence within the system, or indeed outside the system, is the key factor that needs to be considered."
The report is being launched in Dublin later today, with the event to include an address from former UN Special Rapporteur on Torture, Juan Méndez.