Sexton has been recovering from a calf problem, the latest in a string of injuries, and the British and Irish Lions flyhalf's influence was missed by the Irish at Murrayfield last Saturday as they suffered a surprise 27-22 defeat.
Ireland had indicated this week that their experienced number 10 was in contention to return to the team but coach Joe Schmidt chose to retain Paddy Jackson ahead of tougher games against France, Wales and England
"I think it's incredibly disappointing the way we started last weekend. We've certainly given them that challenge this week and hopefully we'll see better things," Schmidt told a news conference on Thursday.
The rejuvenated 33-year-old Ryan, who started both November tests against New Zealand when Ireland recorded their first ever victory over the world champions, returns in place of Iain Henderson who drops out of the matchday squad.
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Schmidt said handing Healy and fellow Leinster prop Jack McGrath a start each at tighthead was always something he planned while winger Andrew Trimble was very close to returning from a hamstring strain but was will also not be risked.
Ireland team: 15-Rob Kearney, 14-Keith Earls, 13-Robbie Henshaw, 12-Garry Ringrose, 11-Simon Zebo, 10-Paddy Jackson, 9-Conor Murray; 8-Jamie Heaslip, 7-Sean O'Brien, 6-CJ Stander, 5-Devin Toner, 4-Donnacha Ryan, 3-Tadhg Furlong, 2-Rory Best (captain), 1-Cian Healy
Replacements: 16-Niall Scannell, 17-Jack McGrath, 18-John Ryan, 19-Ultan Dillane, 20-Josh van der Flier, 21-Kieran Marmion, 22-Ian Keatley, 23-Craig Gilroy
(Reporting by Padraic Halpin, editing by Ed Osmond)
