It built on a nervy opening-day victory against Russia for the hosts, who now top Pool A and are aiming to make the quarter-finals for the first time.
Former Ireland wing Denis Hickie said: “For the host nation to beat the number one ranked team and to win this early on, it will ignite the tournament.”
Japan coach Jamie Joseph said his team were “obviously ecstatic about the result”.
The New Zealander said his side had been “preparing for three years” so “we felt like had an advantage”.
“You’ve just got to be careful before the game. You don’t want to come across too arrogant and cocky, “he added.
” We obviously had a lot of belief in our gameplan and in what we wanted to do. We knew how good Ireland were and how strong they were. “
There was a raucous atmosphere inside the Shizuoka Stadium
The result comes four years after Japan famously shocked the rugby world by beating two-time world champions South Africa at the 2015 World Cup. That 34 – 32 success was dubbed the “Miracle of Brighton” after Japan slid over for an injury-time winning try.
But despite growing enthusiasm for rugby in the country, few would have expected the hosts to repeat the trick against an Ireland side who came into the tournament as the world’s top-ranked team, although went into the match in second behind the All Blacks after their win over South Africa.
Japan also had to recover from trailing at half-time on Saturday, just the second time in 24 attempts they have fought back to win after behind at the break at a World Cup, with the other occasion the win over the Springboks four years ago.
Japan’s stand-in captain Pieter Labuschagne told BBC Radio 5 Live. “We backed ourselves going into this game. That was a great game [against South Africa], but we came into this tournament with a new goal and knew what we wanted to do.
“We are really happy. It is difficult to put it into words but I’m proud of every man and it was a great team effort. “
It was the first time Ireland have lost to a non-tier one nation at the World Cup, with Japan, who had never previously beaten the Irish, having now won five of their last six matches at the tournament.
Japan’s win is ‘so massive’
Former Australia international Matt Giteau, who now plays for Japanese side Suntory Sungoliath, said the win was “so massive for the World Cup”, while former Ireland wing Shane Horgan said it was “no fluke”.
Horgan added on BBC Radio 5 Live: “This tournament needed a second-tier nation to beat a first-tier nation and this was no fluke – they thoroughly deserved it, [they] out-played Ireland.”
Denis Hickie said: “It’s a huge country and rugby is not anywhere near the top sport, so it’s perfect timing.”
Ben Ryan, who coached Fiji to Olympic sevens gold, said it was a “seismic” win, and that he was impressed by Japan’s approach to the game.
“This was textbook from Japan – any young kids watching who want to see how to tackle properly should watch this Japan team,” he said.
Ireland coach Joe Schmidt said: “Congratulations to Japan. What a furious and intense effort it was. We knew it was potentially coming. They are a tremendous side and did really well. “
Can Japan reach the knockout stages?
Japan have never reached the World Cup quarter-finals
Many thought Pool A would be a battle between Ireland and Scotland to see who would face either New Zealand or South Africa in the quarter-finals.
But with two wins from two, it is Japan who top the group and have set a platform to reach the last eight for the first time.
Hickie said the group was now “wide open”, with ex-Scotland scrum-half Rory Lawson in agreement.
“Japan had the pressure of being hosts in their first game, but tonight they showed they are the real deal and mean business in this tournament,” he told BBC Radio 5 Live.
“Their goal of making the quarter-finals is not just a whim. They deserved the win and have blown that group wide open.”
However, with games against Samoa and Scotland still to come, the Brave Blossoms will need to continue their winning form if they are to set up a mouth-watering match with either the All Blacks or another meeting with South Africa in the last eight, while bonus points could play a key part in who progresses.
Ireland should bounce back against Russia next time out, but no side has ever won the World Cup having lost a game in the tournament.
Hickie said: “We saw a situation last time when Japan won three games and didn’t progress. They have their destiny in their hands and Ireland will have to hope Japan and Scotland drop some points.”
Horgan added: “We have an absolute stonker for the last game of this group, Japan versus Scotland. The whole world will be watching, which is exactly what you want at a World Cup.”
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