28-year-old Valbon - originally a replacement for the injured Damien Traille (ankle) - suffered the blow yesterday, forcing coach Bernard Laporte to promote the untried Baby, 21.
Perpignan's David Marty comes onto the replacements' bench.
Team manager Jo Maso confirmed: "We are sticking to our strategy of trying to bring in the maximum number of young players to boost the squad."
The late doubts surrounding prop Sylvain Marconnet and replacement hooker William Servat were dispelled by successful medicals at Paris' La Clinique des Sports.
Baby's Toulouse team mate Fabien Pelous will again skipper the French in Dublin, on the occasion of his 100th cap - as the 31-year-old joins a select band of the international game's centurions.
SIX NATIONS: Saturday, March 12Ireland v France, Lansdowne Road, 1.30pm
(15) Julien Laharrague (Brive)(14) Cedric Heymans (Toulouse)(13) Yannick Jauzion (Toulouse)(12) Benoit Baby (Toulouse)(11) Christophe Dominici (Stade Francais)(10) Yann Delaigue (Castres)(9) Dimitri Yachvili (Biarritz)(1) Sylvain Marconnet (Stade Francais)
(16) William Servat (Toulouse)(17) Pieter De Villiers (Stade Francais)(18) Pascal Pape (Bourgoin)(19) Gregory Lamboley (Toulouse)(20) Pierre Mignoni (Clermont)(21) Fridiric Michalak (Toulouse)(22) David Marty (Perpignan)
Referee: Tony Spreadbury (England)Touch judges: Paul Honiss (New Zealand), Nigel Whitehouse (Wales)