Dublin - South Africa-born Jean Kleyn who only became eligible to play for Ireland in August edged out veteran second row forward Devin Toner when head coach Joe Schmidt announced his Rugby World Cup squad on Monday.
Kleyn, 26, won his first cap against Italy on August 10 two days after he became eligible through the three year residency rule.
Toner can feel hard done by, the 33-year-old Leinster stalwart -- a key figure in the 2018 Six Nations Grand Slam campaign -- starting 50 of Schmidt's 67 Test matches as Ireland boss.
World Rugby vice president Agustin Pichot joined the debate about Toner's controversial omission, suggesting he should ask the governing body, World Rugby, "for answers".
Pichot, who played for Argentina as a scrumhalf, tweeted:"If I was Devin Toner...I will be asking WR for answers.
"I feel sorry for him; nothing against who was properly selected for the RWC19 by the way. But feel that way."
Also missing out for Ireland was scrumhalf Kieran Marmion, who last November was praised for his marshalling of the Ireland team that recorded a historic home win over world champions New Zealand.
Leinster's Luke McGrath goes as back-up to first choice Conor Murray instead -- much is expected of the latter's half-back partnership with world player of the year Johnny Sexton.
Sexton is due to play for the first time in the fourth and final World Cup warm-up match on Saturday in Dublin against Wales.
Grizzled hooker Rory Best -- who showed a return to some form in Saturday's 22-17 win over Wales after a much criticised performance in the 57-15 thrashing by England -- will captain the side in what will be his fourth World Cup.
Sexton and Murray will be going to their third as will fullback Rob Kearney, utility back Keith Earls, prop Cian Healy and second choice hooker Sean Cronin.
Schmidt, who caught people on the hop having said he would make the squad public only after the match with Wales, said it had been a tough call in several positions but he says he wants to strike the right balance which he admits he did not in 2015.
"There were some guys who obviously had more experience and probably had more credit in the bank and were more established," Schmidt told IrishRugby TV.
"There were other guys who were trying to force their way into the group and trying to get a balance of current form versus previous performance, it's always a very very difficult conundrum to try to solve.
"It was one of those typical selection meetings where maybe 20-25 of the players are listed straight away.
"It's those ones where you are trying to get balance where it's very difficult to choose between two players and two players offer slightly different things."
Schmidt is desperate to bow out as head coach with an improved showing from 2015 when an injury-hit side were blitzed by Argentina in the quarter-finals -- the Irish have never got further than the last eight.
They begin their Pool A campaign on September 22 against Scotland, generally regarded as their main rivals for top spot although hosts Japan could have a say in that too and the Irish face them on September 28. Russia and Samoa make up the group.
Forwards: Rory Best (capt), Tadhg Beirne, Jack Conan, Sean Cronin, Tadhg Furlong, Cian Healy, Dave Kilcoyne, Iain Henderson, Jean Kleyn, Peter O'Mahony, Andrew Porter, Rhys Ruddock, James Ryan, John Ryan, Niall Scannell, CJ Stander, Josh van der Flier
Backs: Bundee Aki, Joey Carbery, Jack Carty, Andrew Conway, Keith Earls, Chris Farrell, Robbie Henshaw, Rob Kearney, Jordan Larmour, Luke McGrath, Conor Murray, Garry Ringrose, Jonathan Sexton, Jacob Stockdale
Coach: Joe Schmidt (NZL)
If I was Devin Toner ..... I will be asking WR for answers. I feel sorry for him; nothing against for who was properly selected for the RWC19 by the way. But feel that way.— agustin pichot (@AP9_) September 2, 2019