Internal Garda documents show that senior gardaí gave instructions not to execute warrants against Marie Farrell, a key witness in the Sophie Toscan Du Plantier murder investigation.

The documents, obtained by RTÉ's This Week, show that a Garda sergeant based in West Cork stated that he had been instructed not to execute a number of warrants against Ms Farrell.

Ms Du Plantier was found dead in west Cork in 1996.

The letters were written by Schull-based Gerard Prendeville to colleagues at Bantry Garda Station, over a period of years from 1999 onwards.

In the memos, Sgt Prendeville expressed concern about aspects of the practice.

He outlined how in total, 11 warrants to the value of more than £1,200 (€1,500) were not executed in relation to Ms Farrell.

There is no suggestion of any wrongdoing by Sgt Prendeville, who the letters show was acting on instructions.

The letters were written during the early years of the murder investigation.

Ms Farrell has alleged that during this time she was being put under pressure by other gardaí to wrongly implicate Ian Bailey in the 39-year-old Frenchwoman's murder.

In the memos, Sgt Prenderville said he had struggled to obtain any receipts or acknowledgment for the warrants after he returned them unexecuted to Bantry, as per the instructions he had been given.

He warned that this was unacceptable from an accounting perspective, and he also cautioned that it was likely that the practice would ultimately be queried.

In a separate statement, Ms Farrell also alleged that certain members of the force who were putting her under pressure to implicate Mr Bailey in the murder, had allegedly told her that a number of summonses and warrants would be looked after.

It is believed the bulk of the warrants related to alleged minor motor vehicle offences.

She made the allegation in a lengthy statement to gardaí in 2006, a copy of which has also been seen by This Week.

Ms Farrell's allegations, and the internal memos written by Sgt Prenderville, were all collated as part of an internal garda inquiry which was carried out between 2006 and 2007 under Assistant Commissioner Ray McAndrew.

The so-called McAndrew Inquiry was established on foot of allegations made by Ms Farrell.

He reviewed thousands of files and documents in relation to the case and also took a large number of statements from serving and retired gardaí.

His final 370-page report has never been published.

It is also understood that a named high-ranking garda, who is now retired, gave a statement to the McAndrew Inquiry in March 2007 in which he said that the decision not to execute warrants against Ms Farrell was taken by a more senior named officer.

The Garda press office said it would not be appropriate to comment at this time on the basis that Mr Bailey was suing the State amid claims that an attempt was made to frame him for the murder.

A separate Commission of Investigation is to examine a set of recently discovered tape-recorded phone calls involving, among others, gardaí and Ms Farrell.