Inmates jailed for less than five years should have their sentences halved to tackle chronic overcrowding, campaigners have claimed.
A parole board should also be appointed to decide when convicted killers jailed for life are freed, instead of the minister for justice.
The measures were called for by the Irish Penal Reform Trust (IPRT) and detailed in its position paper on reform of remission, temporary release and parole, launched at the Distillery Building near the Four Courts.
Liam Herrick, IPRT executive director, said a smaller prison population would result in more inmates getting better rehabilitation before their release back into the community.
"At present the biggest obstacle to the prison system operating effectively is the chronic overcrowding throughout the system," he said.
"At the same time, for many prisoners there are no clear guidelines as to what they must do within prison to address their offending in order to move towards release.
"While recent efforts to divert minor offenders away from prisons are important, our current system of deciding when prisoners are released is most urgently in need of change."
There are an average 4,300 prisoners in jails across the country each night, but the inspector of prisons stated there is room for less than 4,000.
Mr Herrick said systems in place at the moment do not help prisoners or society.
"IPRT believes that our current systems of remission, temporary release and parole afford too much discretion to Government and should be replaced by more open and transparent systems of release, in line with the principles of due process and fairness," he added.