Ireland's Sexton out of Six Nations opener
Sexton's calf injury, suffered playing for Leinster against French club Castres, has not yet healed.
Meanwhile, Munster loose forward O'Mahony has yet to regain full fitness following a hamstring problem.
Ulster wing Andrew Trimble remains doubtful with a tight groin.
"It's better to leave him [Sexton] out this weekend," Ireland forwards coach Simon Easterby said Tuesday at the squad's Carton House training base near Dublin.
"We don't want to compromise his calf any further."
In Sexton's absence, Ulster's Paddy Jackson is set to start in the No.10 shirt and direct operation from flyhalf, with Ian Keatley on the bench.
He previously led the backline against Australia in November.
"Paddy has proven himself over a number of months, going back to the summer [June], that he can control the side and dictate the play," said Easterby. "Johnny is one of the best 10s in the world, but we have to adapt.
"That's the mark of this team, we can lose players in key positions and can adapt.
"With Johnny, it's purely now managing his return to training, that will take as long as it takes.
"There's no timescale, felt tightness yesterday and felt it was in Johnny's best interests not to push it.
"It's a case of monitoring or managing it."
Ireland coach Joe Schmidt does have plenty of back-row cover for O'Mahony, with the fit-again Sean O'Brien, Christiaan Stander and Josh van der Flier all available to pack down with experienced No.8 Jamie Heaslip.
Agence France-Presse
