The report said a large number of original statements are missing
A report from the Garda Síochána Ombudsman Commission (GSOC) has found there was evidence of a lack of administration and management of aspects of the investigation into the murder of Sophie Toscan du Plantier.
However the report said there was there was "no evidence of the high-level corruption" as alleged by Ian Bailey, Jules Thomas and Marie Farrell.
The allegations included that Mr Bailey and Ms Thomas were falsely arrested for the 1996 murder of the French woman.
An allegation was also made that Ms Farrell was intimidated into making false complaints against Mr Bailey - including signing blank witness statements, that information from the garda investigation was leaked to the media and that senior garda officers attempted to influence the DPP to bring a prosecution against Mr Bailey.
In its report, GSOC said a "significant number of witnesses were approached" and fully co-operated with the investigation.
Many of these witnesses were local residents living in the West Cork area, who provided circumstantial evidence in relation to Mr Bailey's actions.
"The circumstantial nature of the evidence from the residents was such that Ian Bailey was treated as a significant suspect", it said.
"None of the local Cork residents who co-operated with GSOC indicated that the Garda Síochána were in any way corrupt in their investigation and that witnesses were under any pressure to provide witness statements naming Ian Bailey."
GSOC also said journalists who had reported on the murder and had co-operated with the investigation "had no complaint about the garda murder investigation."
But it noted that: "One journalist was not prepared to name a source of information to GSOC which ultimately had resulted in the alerting of a photographer about Ian Bailey’s arrival to Bandon garda station when he was arrested on 10 February 1997."
The report said that while there was evidence of "a lack of administration and management" with aspects of the investigation, "there was no evidence of the high-level corruption by gardaí".
It said: "While it does certainly appear that journalists were in possession of information in advance of Ian Bailey’s arrests, GSOC was unable to establish the source of the media's information."
And it found that "a large number of original statements and exhibits relating to the murder investigation are missing".
GSOC said this is a matter of "grave concern."
On this issue, the report said: "It is GSOC’s view that a lack of administration and management are the likely explanation for this state of affairs.
"GSOC found no evidence of corruption.
"As a result of the examination of material conducted during this investigation, it is GSOC’s view that it does appear that journalists were in possession of sensitive information about the murder at the time of the garda murder enquiry.
"While there was general cooperation from garda members during the course of the GSOC investigation, a number of garda members were less than cooperative and thus it was not possible for GSOC to fully establish some of the details pertaining to the arrests of Ian Bailey and Jules Thomas."
It added that "pages missing from the original garda 'Jobs Books' in relation to the garda murder investigation are of the most concern to GSOC.
These books form a complete record of all activity undertaken in respect of a major or critical incident (or investigation), along with the rationale for the decisions made.
"This concern is compounded further by the fact that the specific pages missing are from an area of the book when Ian Bailey seems to have first been identified as a potential suspect in the murder by gardaí - and as such, they are potentially very significant."
It added that: "The books are hard-backed in nature, A4 in size and the pages are retained in the book by way of a glued-in spine. As a result, it would not be possible for pages to simply fall out of the book by accident and for them to be removed, this would have to have been a deliberate act."
The report also found that in the witness statements obtained from retired garda members who worked on the original murder investigation, "there was no evidence provided to suggest any garda corruption or malpractice.
"They all described Ian Bailey as being treated as a suspect owing to his eccentric and previously reported violent behaviour."
The witnesses also described Ms Farrell as "allegedly attention seeking and not credible."
"They did not support her allegations of blank statements being signed and indicated that Marie Farrell had willingly co-operated with the murder investigation and had identified Ian Bailey as the male she had seen near to the murder scene at Kealfadda Bridge at the material time."
On garda officers, the report concluded: "Whilst all the senior garda members who were involved in the murder investigation co-operated with GSOC, there were a number of garda members, principally those of detective rank, who did not and thus it was not possible to fully establish some details pertaining to the arrests of Ian Bailey and Jules Thomas which had been the subject of complaint."
There were also a number of retired garda members who died during the course of the investigation and before GSOC was able to speak to them in order to obtain witness statements.