A question mark was placed over Ryan’s availability after the Irish Rugby Football Union revealed the issue earlier in the week but the Leinster lock will take his position in the second row alongside Ultan Dillane.
Ryan will also lead the side out, with Sexton joining Cian Healy and Keith Earls in resting for Saturday’s fixture as well as next weekend’s clash against the United States, while seven players are on British and Irish Lions duty.
Only full-back Hugo Keenan, wing Jacob Stockdale, prop Dave Kilcoyne and flanker Josh Van Der Flier have been retained from the Ireland side that last took the field in a 32-18 win over England in the Six Nations in March.
Stuart McCloskey will earn his fifth cap as he joins Chris Farrell at centre for a fixture where around 3,000 spectators are expected in Dublin, the first time fans have been allowed into the stadium since February 2020.
Peter O’Mahony starts at blindside flanker, with Caelan Doris at number eight, while Ronan Kelleher and Finlay Bealham will sit alongside Kilcoyne in the front row.
Gavin Coombes is among the replacements – joining Rob Herring, Ed Byrne, John Ryan, Ryan Baird, Craig Casey, Billy Burns and Shane Daly – and the Munster back rower is the only uncapped member of the 23-strong squad.
Ireland may be without a host of regulars but Japan head coach Jamie Joseph is taking nothing for granted.
Joseph said: “The Irish rugby team and the Irish people are very proud people so we’re expecting a huge challenge, particularly at the start of the game, we think they will bring the game to us.
“It’s a good opportunity for some of the younger players, with the Lions away, to make a challenge and a bid for the jersey.
“It doesn’t really matter who they have playing, they’re going to be a lot more experienced at Test match rugby than we are.”
Ireland were memorably defeated by Japan in their most recent encounter at the 2019 World Cup pool stage, but Joseph believes the Brave Blossoms’ opponents are a more formidable force on home soil.
He said: “We know that at home they’re very strong, a lot stronger than when they’re away, so it’s a different kettle of fish and that’s something that we’ve been consistently talking about all week.”
Joseph has made three changes to the side that were beaten 28-10 by the Lions at Murrayfield last weekend, with the uncapped Semisi Masirewa coming on to the wing, allowing Kotaro Matsushima to replace Ryohei Yamanaka at full-back.
Naoto Saito is set to feature at scrum-half while Kazuki Himeno will be Japan’s number eight.