Marguerite Bouniol, 85, and her 87-year-old husband Georges touched down at Cork Airport on board an Aer Lingus flight from Paris just before 3.30pm.
Accompanied by Jean Pierre Bloc, from Assoph, the group campaigning for the truth about the murder of Ms du Plantier, and his wife, Marie Paul, they were greeted in the arrivals hall by Françoise Letellier, the honorary French consul in Cork, and Superintendent Liam Horgan, the Garda liaison officer.
Mrs Bouniol, who was ill over Christmas, looked frail.
The couple were happy to greet and speak briefly to reporters but they declined to comment publicly on their visit.
A family friend said they were anxious to avoid commenting about any aspect of the French investigation into their daughter’s death while legal proceedings were ongoing in Ireland.
The group travelled to Sophie’s holiday home in Toormore, near Schull in West Cork, where her badly battered body was found on Dec 23, 1996.
They will attend an anniversary Mass in Goleen at the weekend, before returning to France early next week.
Mr and Mrs Bouniol were too ill to travel to Ireland last December to mark the actual date of their daughter’s death.
They instead marked the anniversary in Lozere in France, where Sophie’s remains are buried.
Their visit comes as a judgment is awaited in the Supreme Court challenge by law graduate and former journalist Ian Bailey against his extradition to France in connection with the French investigation into Sophie’s murder.
Mr Bailey, aged 54, has always denied any involvement in her death.