The States four main prisons are 245 inmates over capacity, according to figures obtained by Fine Gael today.
Justice spokesman Jim O'Keeffe revealed the figures after a 28-year-old man was the fifth prisoner to have been the victim of a violent attack in an Irish prison in the past week.
He said the figures show that Mountjoy, where four of the attacks took place, had 57 more inmates than its 440 capacity. Wheatfield was overcrowded by 52 and Castlerea, Co Roscommon was 25 over capacity.
Cork Prison, where a man on remand for murder was attacked on Sunday, was undergoing the highest level of overcrowding with 261 inmates in a prison designed to house 150.
The figures suggest the four main prisons exceed capacity by 23 per cent.
Prison overcrowding surfaced as an issue this week when Gary Douch (21) was beaten to death in a cell in the early hours fo Tuesday morning after being put there after seeking protection. There were six other inmates in the cell and the chief suspect was recently assessed in the Central Mental Hospital.
Mr O'Keefe said overcrowding was "a clear factor in the recent Mountjoy murder".
Minister for Justice Michael McDowell ordered an independent inquiry into the killing but Mr O'Keefe said his policies which included the closure of two prisons in recent years had led to the overcrowding.
"The Minister's response to the murder of Gary Douch, a man who was effectively in his care, was to recommend that single cells should be made available for any prisoner concerned about their safety.
"This would be a welcome development, if it were possible. But Mountjoy Prison does not have any vacant single cells because it is already totally overcrowded," Mr O'Keefe said.
Mr McDowell has used prisons "as a pawn in his battle with the prison officers," he added.
Labour Party justice spokesman Brendan Howlin said today's stabbing suggested "a serious breakdown of order and control within the country's biggest prison".
"Political responsibility for this shocking situation must rest with the Fianna Fáil/PD Government which has been in power since 1997 and particularly with the current Minister for Justice McDowell who, as with other areas of his portfolio, has failed to deliver on the lavish promises he has made," Mr Howlin said.
"The result of this is that prisoners still have to endure severely overcrowded and degrading conditions. When these are combined with the record temperatures we have experienced over recent weeks, it creates a potentially explosive mixture," he added.
The Inspector of Prisons had warned about "explosive conditions" Mr Howlin said, urging the minister to begin implementing the recommendation contained in Mr Justice Kinlen's last report.