The historic victory has left Japan top of Pool A with a perfect record after two games, while Ireland, so impressive in beating Scotland last weekend, has suffered its first loss.
First half tries from Garry Ringrose and Rob Kearney, his effort converted by Jack Carty, had left the Irish 12-9 ahead at the half, with three penalties from Yu Tamura keeping Japan in touch.
But the second half saw Japan assume total command with wave after wave of attacks, culminating in a try for replacement Kenki Fukuoka, converted by Tamura.
Ireland went into the World Cup as the No.1 ranked team in the world and coach Joe Schmidt was “incredibly disappointed that we didn’t manage to control the end of the game but they are a tremendous side, they did really well,” he conceded.
His side will look to recover against Russia in its next game, while Japan faces tough clashes against Samoa and Scotland in its bid to reach the quarterfinals knockout stage.
For now, Japan coach, ex-All Black Jamie Joseph, could relish a victory that had been long in the making, seen as a crucial clash after the draw for the tournament was made.
“I’m very proud of the players, we obviously had a plan and we’ve thinking about this game for a long time,” he said.
Ireland ahead early
Ireland, who thrashed Scotland 27-3 in its opener, had started confidently in a bid to quieten the highly partisan crowd of 47,000.
It looked like succeeding as Ringrose connected with a clever Carty kick to go over for a try in the corner after just 13 minutes.
Carty, filling the shoes of Ireland’s sidelined flyhalf star Johnny Sexton, was doing a great job with the crucial assist as full-back Kearney dived over seven minutes later, also making the successful conversion.
12-0 down, Japan was wobbling but Tamura kept them in the game with three penalties as the game was beginning to slip away from Ireland.
The introduction of Japan’s regular captain Michael Leitch after half an hour also proved a key factor and although Tamura missed a penalty chance early in the second half, it was not long before the pressure told on Ireland.
Winger Fukuoka supplied the first killer blow, showing brilliant footwork to cross for his side’s sole try and the lead for the first time in the match, extended by the sure boot of Tamura.
The crowd in the Ecopa Stadium erupted as the ‘Shizuoka Shock” was set to be added to the “Brighton Miracle” – the latter the venue when Japan beat the mighty South Africa Springboks at the 2015 tournament.
Ireland huffed and puffed in response, but another Tamura penalty stretched the advantage to seven points and a late Fukuoka burst ensured any chance of a response was nullified.