The 39-year-old wife of celebrated French film producer Daniel Toscan du Plantier was found beaten to death at her holiday home in County Cork, southwest Ireland, on December 23, 1996.
Despite being one of the most high-profile murder investigations in Irish history, no one has ever been charged in relation to her death.
The Association for the Truth About the Murder of Sophie Toscan du Plantier (ASSOPH), an organisation campaigning for justice for the mother-of-one, staged a press conference in Cork city late Saturday.
They denounced the Irish investigation as a "judicial fiasco" and a "denial of justice".
The police investigation has been described as "thoroughly flawed" and "prejudiced" by the then Irish director of public prosecutions.
The victim's mother Marguerite Bouniol was too unwell to attend the Cork press conference but a letter from her stating "enough is enough" was read-out on her behalf by Sophie's uncle, Jean-Pierre Gazeau.
"We, the family, have been suffering a double penalty for almost 18 years: Sophie was murdered and we, the plaintiffs, are mute in Ireland," she said in the letter.
"We never had the opportunity to make our case and now we are victims of serious malfunctions from police and judicial Irish systems."
Briton Ian Bailey, 56, a former freelance journalist who has long lived in Ireland, was twice arrested for questioning about the murder but was never charged.
In 2008 a Paris-based examining magistrate, Patrick Gachon, began a separate inquiry, building an investigation file independently of the Irish police file and in February 2010 issued an arrest warrant for Bailey.
Ireland's Supreme Court has overturned a decision to extradite him. However, a European Arrest Warrant remains in place, which means Bailey is effectively barred from leaving Ireland.
"In no way should this confusion ever hide the substance of the case, namely that Sophie was murdered and the fact that her killer remains free," Jean-Antoine Bloc-Daude of ASSOPH told reporters.
"The judicial actions should never pollute or impact on a determined and sincere search for the truth."
Bloc-Daude said he expected the French investigation to conclude this year.
He said he understood a team of French detectives was due to visit Ireland on Monday to carry out further interviews as part of the Gachon probe.