Racing 92 winger Thomas scored the last of France's five tries in last weekend's 38-21 victory over Wales but has been ruled out of the Ireland game at Stade de France with a hamstring injury.
Fickou shifts out wide in the absence of the clinical Thomas, with Arthur Vincent coming in at centre in the only change from the win over Wales.
Explaining his decision, France head coach Fabien Galthie said: "We know that Jonny Sexton uses his foot to put pressure on our wing. Fickou has this experience to deal with it.
"We had to manage the same thing against Wales. Arthur Retiere is also ready and is on the bench."
France is seeking its first tournament triumph in a decade as it gears up to host the 2023 Rugby World Cup.
Les Bleus start their final game in Paris a point behind leader Ireland and level with England, which faces Italy in Rome.
"It's quite simple - we have as much chance of winning as England and Ireland," Galthie said. "But above all we try to be consistent.
"We want consistency. We should not be weighed down with assumptions of what could happen."
Bernard le Roux again partners Paul Willemse at lock after his citing for an allegedly striking Alun Wyn Jones was dismissed by an independent panel.
The game has been given the all clear to go ahead despite French president Emmanuel Macron tightening the country's coronavirus restrictions this week.
Ireland announced its squad on Thursday, with Robbie Henshaw replacing injured Leinster team-mate Garry Ringrose in their only change.
France: Anthony Bouthier, Vincent Rattez, Virimi Vakatawa, Arthur Vincent, Gael Fickou, Romain Ntamack, Antoine Dupont; Cyril Baille, Julien Marchand, Mohamed Haouas, Bernard Le Roux, Paul Willemse, Francois Cros, Charles Ollivon (captain), Gregory Alldritt.
Replacements: Camille Chat, Jean Baptiste Gros, Demba Bamba, Romain Taofifenua, Dylan Cretin, Baptiste Serin, Arthur Retiere, Thomas Ramos.