Ireland head coach Joe Schmidt has put his faith in Connacht's Jack Carty to start at out-half for Saturday’s crucial World Cup clash with Japan.

With Johnny Sexton struggling to shake off the effects of a bang to the thigh, 27-year-old Carty will make his first World Cup start and only his second ever start in an Ireland shirt. 

Ireland have made four changes in total for the Pool A showdown in Shizuoka. Rob Kearney returns at full-back, while Keith Earls is back on the wing. Chris Farrell comes into the centre in place of the rested Bundee Aki.

In the forwards Jack Conan has had to drop out after he injured his foot in training.

He had been slated to come into the back row for Peter O’Mahony, with CJ Stander switching to number 6, however O'Mahony now starts and Stander will occupy the No 8 jersey. 

Joey Carbery - back from an ankle injury - is named in the replacements, as is Rhys Ruddock.

"Obviously we've left a couple of guys out who trained on Tuesday but not fully and we decided to give ourselves certainty so we told him (Carty) he would be starting on Tuesday so he would get his head around it," said Schmidt.

"I thought he came on and played really well against Scotland. That gives us confidence and just allows us to freshen guys up and work through a few combinations as we work our way through the pool and hopefully it will be a cohesive performance because it’s going to have to be against Japan because we know how good they can be."

Schmidt would never admit it but with a potential quarter-final against South Africa on the horizon, keeping Sexton fresh and healthy is of paramount importance.

"He's never happy about being left out, he’s always looking to play but he had a good hit out against Scotland," the head coach added.

"We thought about bringing him off the bench and giving us the security off it but we also have had a lot time for Joey and we wanted to get him involved as well.

"It just means that we take the pressure off Johnny a little bit this week. Johnny’s a guy who plays really well fresh and can come back in and hit the ground running. We’ve seen that both in Leinster and the national team. I’ve been involved coaching Johnny for almost 10 years so it’s not something I'd be too concerned about." 

Ireland: Rob Kearney; Keith Earls, Garry Ringrose, Chris Farrell, Jacob Stockdale; Jack Carty, Conor Murray; Cian Healy, Rory Best (capt), Tadgh Furlong; Iain Henderson, James Ryan; Peter O'Mahony, Josh van der Flier, CJ Stander.

Replacements: Sean Cronin, Dave Kilcoyne, Andrew Porter, Tadhg Beirne, Rhys Ruddock, Luke McGrath, Joey Carbery, Jordan Larmour