The Brave Blossoms shocked the world with their victory over South Africa in England four years ago and, playing as hosts this time around, they were at it again on Saturday.

Ireland, which started the tournament as the world's top-ranked side, looked to be cruising when Jack Carty excelled in teeing up tries for Garry Ringrose and Rob Kearney.

But Yu Tamura's boot brought Japan back into contention before the break and a gutsy display in the second period was finally rewarded with Kenki Fukuoka's decisive score.

Joe Schmidt's men predictably came again but could find no way through a disciplined home outfit who are now on course to top Pool A and progress to the quarter-finals.

The final whistle sparked scenes of jubilation as Japan's players and fans celebrated a memorable victory on home soil, secured amid a tremendous atmosphere.

Ireland has never been past the last eight and, while it should still advance to that stage, two-time defending champion New Zealand would be its likely opponent if it finishes second in the pool.

After an early Japan flurry, the Ireland pressure built and Carty expertly picked out Ringrose with a crossfield kick for the opening try in the right corner.

Tamura landed a penalty in response, having missed an earlier effort, yet Ireland were now rampant and Carty knocked his own kick down for Kearney to lunge just inside the line for try number two.

However, the hosts rallied when many might have expected them to crumble and a pair of Tamura penalties heading into the break reduced the deficit to 12-9.

Ireland still could not shake off its dogged opponent, which earned another penalty for Tamura from range, only for his boot to let it down this time.

The Brave Blossoms were not to be denied when they next came forward, this time driving Ireland back towards their own try line and finally finding Fukuoka in space on the left for a breakthrough try.

Japan stood firm and another Tamura kick put it a converted try in front with just eight minutes to play, a position it again defended superbly for an historic triumph.

Japan faces Samoa next and then Scotland but will be confident of beating both based on this performance. Ireland at least has a kind fixture in which to recover, facing minnow Russia.