New evidence criticising the garda investigation into the murder of film-maker Sophie Toscan du Plantier will go before the Supreme Court on Monday.
One-time suspect Ian Bailey won a legal challenge to have recently released state documents admitted as part of his appeal against extradition to France, where he is wanted for questioning over the 1996 murder in Cork.
The five-judge Supreme Court ruled the fresh evidence claiming the murder probe was flawed and prejudicial will be part of a three-day appeal, starting on Monday. But they rejected his barrister's application for the entire case to be reheard in the High Court.
Mr Bailey's extradition was ordered by the High Court last summer.
The 54-year-old's legal team had applied for the courts to have access to state documents which say the original criminal investigation was tainted by "garda misbehaviour of the lowest standards".
The new material was provided by the State to Mr Bailey's lawyers last November and included a 44-page critical review of the conduct of the murder investigation and the reasons why the former director of public prosecutions (DPP) decided not to prosecute.
It came to light when Eamonn Barnes, now retired, emailed authorities dealing with the extradition to tell them about the existence of his 2001 review.
The court heard Mr Bailey's legal team had been told the Attorney General, Marie Whelan, felt the material was "very significant" and may be something Mr Bailey will rely on, while Justice Minister Alan Shatter believed that in the interests of justice they should have it.
Mr Bailey, who was in court, denies any involvement with the murder of Ms Toscan du Plantier. The 39-year-old's body was found near her holiday home in Schull, Co Cork, on December 23 1996. He was arrested and questioned twice by gardai, but no charges were ever brought.
Under French law, authorities can investigate the suspicious death of a citizen abroad but they cannot compel witnesses to go to Paris for questioning.