The Brave Blossoms tore through Ireland time and time again in Shizuoka, triumphing through superior tenacity and supreme intensity.
The world’s 10th-ranked team beat the second-best outfit, who entered this tournament ranked number one in the standings.
Wing Kenki Fukuoka ran in the try that sealed Japan’s famous victory, immediately invoking memories of their breathless 34-32 win over South Africa in Brighton in 2015.
The 27-year-old stormed in at the corner to send the Shizuoka Stadium Ecopa into delirium though, and rip up the pre-written expectations for Pool A.
Ireland failed to cope with Japan’s intensity and precision, and this defeat leaves their group wide open.
Joe Schmidt’s men would have expected to move from their potent 27-3 win over Scotland and press through to top Pool A for a quarter-final against South Africa.
If Japan could repeat this level of performance however, it could be the Brave Blossoms who top the pool, with the hosts having already bested Russia 30-10.
That would leave Ireland and Scotland scrapping it out for second place, and a possible quarter-final against back-to-back world champions New Zealand.
But Japan refused to let Ireland gain any control through the dominance of possession, and the hosts’ relentless pace caught boss Schmidt’s men off guard.
Japan will move on to face Samoa in Toyota on Saturday, with a brittle Scotland doubtless now very nervous about taking on the tournament hosts in Yokohama on Sunday, October 13.
Ireland meanwhile will take on Russia in Kobe on Thursday, with much wound-licking to come between now and then.
Tamura missed a regulation shot at goal as Japan opened the clash at a furious pace.
Once Ireland settled, Schmidt’s men set up Japan perfectly for Carty to bag full reward from a crossfield punt.
Ringrose comfortably beat Lemeki to the high ball, fielding cleanly and finishing in style.
Tamura’s first penalty had Japan on the board after Peter O’Mahony was ruled offside at a ruck.
But Ireland bulldozed the Japan scrum, punted the penalty upfield for a good platform – and Carty again outfoxed the home defence with his kicking game.
The 27-year-old chipped over the line, tapped back – and full-back Kearney scooped up to barrel home for the try.
A lost lineout put Ireland under huge pressure as Japan continued to race forward at a hectic pace. Conor Murray was pinged for killing the ball in his own 22, so Tamura posted the facile penalty to cut Ireland’s lead to 12-6.
Another Tamura penalty allowed Japan to cut Ireland’s lead to just three points.
And Ireland were so rattled by the Japanese onslaught that Carty sent the restart dead, handing the hosts a scrum on the halfway line.
Japan stole the chance for one last attack in the half, ploughing through 16 phases with time well and truly up.
But just when the hosts looked to have broken Ireland’s defence, hooker Shota Horie’s grubber kick evaded the onrushing Lomano Lemeki – and Ireland tiptoed to half-time with that slender 12-9 lead.
Japan continued to threaten, until Ireland won a scrum penalty and kicked to the corner rather than for goal. Ireland duly lost the lineout however, wasting the hard-won platform.
Japan kept on coming, time and again, and eventually Fukuoka crossed in the left corner.
CJStander collided with Chris Farrell in a bungled break from the scrum, gifting Japan the scrum deep in the Ireland 22.
The pumped-up hosts lapped up the chance, powering through the middle before whipping the ball wide.
Tamura’s fine conversion then put Japan into a 16-12 lead on the hour.
Ireland emptied the bench and pressed for a score to turn the tide, but it never came.
And when Tamura posted his fourth penalty for a 19-12 lead, Japan’s stunning victory was complete.