Ntamack was banned from Les Bleus' 73-24 demolition of Italy after picking up a red card against Wales and being handed a two-match suspension.
Penaud, who has 37 international tries and is one short of Serge Blanco's all-time France record, also did not play against the Azzurri after a disappointing performance in the 26-25 defeat by England.
"(Ntamack) has been training with us throughout the tournament. He's part of the team dynamic," Galthie told reporters on Thursday.
"Damian is back in the team because he has undeniable potential and he's ready to play against Ireland."
The Irish lead the standings on 14 points, three ahead of France, whose chances of winning the tournament hang in the balance when they visit the Aviva Stadium.
England are third with 10 points ahead of their home game with Italy on Sunday.
"This match has special stakes, we know that and that's what we want," Galthie added of facing Ireland.
"What's more, it's a match against the best European nation in the last three or four years.
"They're a very high-level team, one of the two best nations in the world, if not the best, and they're playing at home.
"It's true that the challenge is immense. And taking on a huge challenge is what we're looking for."
Galthie has also opted to again use a 7-1 forwards/backs split on the bench, which France employed in their win over Italy.
"The choice of a 7-1 bench is linked to the profiles of the most competitive players we currently have in the French team," the 55-year-old said.
"Looking at the performances we've had for a while now, that's what we think is the best way to perform."
France lineup
15-Thomas Ramos, 14-Damian Penaud, 13-Pierre-Louis Barassi, 12-Yoram Moefana, 11-Louis Bielle-Biarrey, 10-Romain Ntamack, 9-Antoine Dupont (captain), 8-Gregory Alldritt, 7-Paul Boudehent, 6-Francois Cros, 5-Mickael Guillard, 4-Thibaud Flament, 3-Uini Atonio, 2-Peato Mauvaka, 1-Jean-Baptiste Gros.
Replacements: 16-Julien Marchand, 17-Cyril Baille, 18-Dorian Aldegheri, 19-Emmanuel Meafou, 20-Hugo Auradou, 21-Oscar Jegou, 22-Anthony Jelonch, 23-Maxime Lucu