Joe Schmidt insists Peter O'Mahony is "mad keen" and fully ready to face Japan just six days after a concussion check.
Jack Conan was meant to start Saturday's World Cup clash against the tournament hosts in Shizuoka but picked up a foot issue in training on Thursday.
O'Mahony has therefore slotted back into the starting line-up, leaving Ireland fielding the same pack that subdued Scotland 27-3 in Yokohama last weekend.
The Munster and British and Irish Lions flanker failed a Head Injury Assessment (HIA) in Sunday's Scotland clash, but passed all the checks afterwards, did not suffer a concussion – and returned to full training on Tuesday.
And that left head coach Schmidt insisting there is no extra risk in fielding O'Mahony against Japan.
"Pete was mad keen to play, he only played that 25 minutes and unfortunately I think we might have got Pete back out there but we had two HIAs at the same time," said Schmidt.
"And therefore the communication was not as smooth as it normally is.
"He flew through his HIA two and HIA three which confirms to us that he didn't have a concussion.
"The HIA process is a precautionary process, so that anyone who has a suspected concussion is removed from the field, it doesn't mean they have had a concussion.
"So there's no elevated risk, we don't believe, in selecting Pete; and we've got a super medical staff.
"With Jack Conan, someone just stood on his foot. He is absolutely fine. But it meant that he was sore in training, he didn't train fully.
"Again it's a case of being confident in the squad, and offering an opportunity.
"And I'm pretty sure Jack will be fine to train tomorrow, but again we rolled our sleeves up today and worked pretty hard so we wanted to get that team to have the chance to play."
Jack Carty will make just his second Test start when Ireland face hosts Japan in Saturday's World Cup clash in Shizuoka.
The Connacht fly-half will pilot Ireland's backline with boss Schmidt not willing to risk Johnny Sexton, who has nursed a minor thigh issue.
Joey Carbery is fit again after ankle ligament trouble and takes a seat on the bench.
Carty impressed off the bench in the closing stages of Ireland's bonus-point win over Scotland in Yokohama, and now the eight-cap playmaker has the chance to impose his presence on this World Cup.
"Jack's very calm, he doesn't get ruffled easily, he doesn't get distracted by an error he might make, or people trying to put pressure on him," said Schmidt, of Carty.
"So he stays in the zone very well.
"Johnny's never happy about being left out, he's always looking to play, but he had a good hit-out against Scotland, played through 55 minutes and got a bit of mileage under his belt there.
"We thought about his security off the bench but we also have a lot of time for Johnny and wanted to get him involved as well.
"It just means we take the pressure off Johnny this week."
Sexton ceded goal-kicking duties to Conor Murray early in Ireland's win over the Scots, and was withdrawn straight after his side's bonus-point try.
The Leinster star did not train fully on Tuesday, and Schmidt has now chosen not to take any chances with the 2018 World Player of the Year.
Rob Kearney and Keith Earls return to the starting XV having missed the Pool A opener against Scotland due to injury, with Ireland making four changes to their starting line-up.
Kearney has shaken off his calf complaint and Earls is now over his own thigh concern.
While Bundee Aki also completed all the head injury checks and returned to full training, head coach Schmidt opted not to risk Connacht's battering ram centre.
So Chris Farrell starts at inside centre, with the Munster man in fine form and fettle in Japan.
Ireland team to play the Japan in Pool A of the World Cup on Saturday, September 28 (0815 BST KO):
R Kearney (Leinster), K Earls (Munster), G Ringrose (Leinster), C Farrell (Munster), J Stockdale (Ulster), J Carty (Connacht), C Murray (Munster), C Healy (Leinster), R Best (Ulster, capt), T Furlong, J Ryan (both Leinster), I Henderson (Ulster), Peter O'Mahony (Munster), J Van Der Flier (Leinster), CJ Stander (Munster).
Replacements: S Cronin (Leinster), D Kilcoyne (both Munster), A Porter (Leinster), T Beirne (Munster), R Ruddock, L McGrath (both Leinster), J Carbery (Munster), J Larmour (Leinster).
