New figures released to the party revealed that the overall occupancy rate in the country's prisons is 105%.
The worst overcrowding is at the Dóchas women's prison in Dublin where there are 108 inmates, but the bed capacity is 85.
Mountjoy is 17% over its capacity, while Castlerea has 13% more prisoners than it should.
Fine Gael Justice Spokesman Charlie Flanagan, who got the figures from a parliamentary question to the Minister for Justice, said overcrowding creates tensions and makes prisoner violence more likely.
It also serves to hinder the rehabilitation capacity of the prisons, he said.
He called for action to stop people who fail to pay fines being sent to prison, to ease the pressure.
In response, a spokesman for the Minister for Justice said that 400 new prison places would be coming on stream in the next few months.
The spokesman also said planning is continuing for the creation of the Thornton Hall super prison in north Dublin, which will have over 2,000 places.
He said there had been record investment in the prison system by the Government, and added that the growing number of inmates reflected the increased success of the expanded Garda Síochána, and the higher custodial sentences being handed down by the courts.
