Joe Schmidt's side, who came into the tournament as the world's top-ranked team, crushed Scotland 27-3 with a clinical performance in their Pool A opener last weekend.
Japan thrashed Russia 30-10 in last Friday's (Sept 27) World Cup curtain raiser, sparked by a hat-trick from Kotaro Matsushima, one of several lightning Japanese wingers head coach Jamie Joseph calls his fleet of "Ferraris".
The Brave Blossoms have one way of playing - fast - and will look to shift the ball wide to Matsushima and William Tupou as quickly as possible in Shizuoka on Saturday (Sept 28).
Ireland though are banking on their rush defence in midfield to smash those supply lines, just as they did against Scotland when Chris Farrell, Bundee Aki and Garry Ringrose ran riot.
Expect Ireland to swarm over all Japan at the breakdown, where pint-sized scrum-half Yutaka Nagare will see way more of Josh Van Der Flier than he would probably wish.
With Sexton out, Jack Carty takes over at fly-half in only his second Test start, but Schmidt insisted he was more than up to the task.
"He came on and played really well against Scotland," said the Kiwi-born coach, who resisted the temptation to make wholesale changes and stuck with 11 of the side who proved too strong for their Celtic rivals.
"He cleared our lines really well and the distance of his angled kicks was spot on. Hopefully it will be a cohesive performance from us, because it will have to be."
- Element of surprise -
Japan broke into the world's top 10 under Joseph's predecessor Eddie Jones, and have become almost a permanent fixture there under former All Black Joseph.
But they no longer carry the element of surprise following their heroics at the last World Cup four years ago, when they produced the biggest upset in the tournament's history by stunning South Africa 34-32 in their opening game on the England's south coast.
The Japanese, then coached by Jones, went on to record two further victories, although they narrowly failed to reach the quarter-finals -- the target Joseph has set them this time around.
Joseph has sprung a surprise by relegating his marauding captain Michael Leitch to the bench for Saturday's (Sept 28) game, meaning Pieter Labuschagne leads Japan for the first time.
Ireland, who themselves have never won a World Cup knockout game, are likely to prove too big a hurdle.
But a defiant Labuschagne promised Japan would not wilt under the pressure of being the tournament hosts.
"It brings the best out of you," said the Pretoria-born flanker. "The whole team embraces it - it just makes you feel alive."
Don't forget to grab this week's issue of The Phuket News for your free Rugby World Cup wall chart, listing when all the big matches will be played!