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 found that the majority of prisoners on such 'restricted regimes' are there for self protection.
The study says tackling problems of violence in our prisons will reduce the numbers in isolation.
The report recommends: "Separation of a prisoner from others should not be permitted for reasons of punishment, but only for reasons of safety in emergency situations, and for the shortest possible period of time."
The report was launched in Dublin today, with the event including a keynote address by former UN Special Rapporteur on Torture, Juan Méndez.
Speaking ahead of the launch, Deirdre Malone - Executive Director of the Irish Penal Reform Trust - said: "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."
She added while their initial goal is the abolition of solitary confinement, they are ultimately hoping to see "12 hours out-of-cell time daily for all prisoners".