Fianna Fáil leader Micheál Martin said he has seen evidence alleging Gardaí failed to properly investigate serious crimes including murder and abduction.
Martin said he obtained the documents after meeting Garda whistleblower Maurice McCabe.
One of the cases in question is believed to be the murder of Sylvia Roche Kelly in Limerick in December 2007 by Jerry McGrath.
The widower of Sylvia Roche Kelly filed suit against the State in 2010 over the fact that McGrath was out on bail at the time.
It's understood he will argue the liability of the State as he believes McGrath should not have been granted bail.
The case is due before the High Court on Monday.