The Garda Ombudsman has criticised the force for failing to keep exhibits and documents safe and available for future use in a criminal investigation.
The criticism is contained in a report issued following a complaint about the original garda investigation into the death of Fr Niall Molloy.
The complaint was made by Henry McCourt, nephew of Fr Molloy, who died in Clara, Co Offaly, on 8 July 1985.
One person, Richard Flynn, was charged in relation to the death, but was acquitted of manslaughter on direction of the trial judge in June 1986.
Since that time, the unsolved death has been considered to be an open investigation.
An Garda Síochána has said it will examine the report's findings to see what lessons can be learned from it.
In a statement, the force added that as part of its modernisation and renewal programme has introduced PEMS (Property and Exhibit Management System) to ensure the proper recording and storage of all exhibits.
In 2013, the Garda Serious Crime Review Team published its report into the death of Fr Molloy.
The report examined the original investigation and reached a number of conclusions.
The outcome of the SCRT report led to calls for further investigation into the death and circumstances surrounding the garda investigation at the time in 1985.
In 2014, Senior Counsel Dominic McGinn was appointed by the then minister for justice to review the contents of the SCRT report.
Mr McGinn found that in light of the passage of time and the death of key witnesses, further investigation would not be likely to establish what had occurred leading to the death of Fr Molloy.
Watch:
- Inquest launched into death of Fr Niall Molloy (RTÉ Archives)
- Developments in Molloy case ( RTÉ Prime Time)
He also agreed with the SCRT findings in relation to the deficiencies in the original investigation.
At this point, Mr McCourt contacted GSOC to investigate the complaint about the deficiencies in the original investigation.
GSOC said the force's failures were not isolated to the particular facts of this investigation and had arisen in other cases.
It said it had raised these matters with the Garda Commissioner and this is set out in the report released today.
In its report, GSOC said that under the Garda Síochána Act 2005 it is not able to investigate the actions of retired members of the force as they can no longer be held liable for breaches of discipline that may have arisen when members of the force.
It added that it is "of real concern" that exhibits and documents cannot now be found.
