Some 470 prisoners were released early from Cork Prison last year, it has emerged.
The inmates were freed before they completed their sentence and before they were entitled to remission, or time off for good behaviour.
Fine Gael, who obtained the figures from the Department of Justice, said the prison service was now so overcrowded due to prison closures that it was returning to the revolving door system.
A spokeswoman for Minister for Justice Michael McDowell said 65 per cent of the 470 released inmates were in the last month of their sentence. She said the releases were part of structured and planned sentence management and were not related to overcrowding.
Fine Gael's spokesman on justice Jim O'Keeffe TD said the figures supported anecdotal evidence he had been hearing that Cork Prison was so overcrowded inmates were being freed early to make way for newly-convicted prisoners. "The figure is amazingly high," he said.
"It is directly related to the closure of Spike Island prison in Cork without planning for the likely consequences. Michael McDowell has talked of building a new prison to take its place but that is a long, long way off.
"He claims prisoners have been released early as part of their reintegration into society.
"But my information is that when a prison van is bringing newly-convicted criminals to Cork Prison the number of beds they are going to occupy is simply freed up by letting other inmates out early."
Reports in recent years from the Cork Prison visiting committee and the Inspector of Prisons, Mr Justice Dermot Kinlen, have consistently pointed out that the prison holds between 250 and 280 inmates despite being designed for 150.
Mr O'Keeffe obtained the early release figures after he tabled a parliamentary question. He said he had tabled further questions in relation to early releases in other prisons in an attempt to determine if the revolving door system was operating across the prison service.
In his reply relating to Cork Prison, Mr McDowell does not refer to early releases, opting instead to use the term "temporary release" to describe the status of prisoners released early.
Later in his reply he also uses the phrase "temporary release" to refer to prisoners who have been freed for a short period, usually several days and often for family reasons, but must return to the prison when that period is over.
This is the traditional meaning of the phrase "temporary release" in the prison context. There are currently 111 prisoners on temporary release.
The Minister's spokeswoman said this figure represented 3 per cent of the prison population compared with the 20 per cent of inmates on temporary release 10 years ago.
In his response to Mr O'Keeffe, Mr McDowell explains the early release of inmates before their remission period saying:
"Temporary release arrangements operate similarly to a system of parole, which is a feature of prison systems worldwide. They are an important vehicle for re-integrating an offender into the community in a planned way."
Mr O'Keeffe said the Cork Prison was so overcrowded and early release rates were so high that the releases could not be planned.