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.
Its new report has found that the majority of prisoners on such "restricted regimes" are there for self-protection.
Executive Director of the trust is Deirdre Malone who says tackling violence in our jails should reduce the numbers in isolation.
She said: "Really there now needs to be a concerted effort to reduce violence in prison 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".