Shizuoka - Kenki Fukuoka struck the killer blow as hosts Japan stunned Ireland 19-12 on Saturday to take a giant step towards a first Rugby World Cup quarter-final spot.

The replacement winger wriggled over on the hour mark to score the winning try of a frenetic Pool A clash as the Brave Blossoms added the "Shizuoka Shock" to the "Brighton Miracle" after toppling South Africa at the 2015 tournament.

Japan coach Jamie Joseph insisted the result had not come as a surprise.

"You don't want to come across too arrogant and cocky," said the ex-All Black.

"We'd been preparing for this game for a hell of a lot longer than they had," added Joseph.

"We were preparing for three years and Ireland probably since Monday, so we felt like had an advantage. The boys will have a couple of beers tonight."

Ireland coach Joe Schmidt, meanwhile, denied that his side had underestimated Japan.

"It's a tough defeat for us to take," he admitted. "But I'd like to congratulate Japan for the energy, intensity and skill they brought to the game.

"Japan didn't exceed my expectations -- I expected them to be as good as they were. They play a quality game and they're very difficult to contain."

Ireland, who thrashed Scotland 27-3 in their opening game, had looked in control after first-half tries from Garry Ringrose and Rob Kearney.

But they went off the boil as Japan grew into the match, roared on by a partisan crowd of 47,000.

A late inclusion on the bench, Fukuoka made the difference, darting over on the overlap to complete a stunning team try and put the home side in front.

Incredibly, they stayed there, leaving Ireland's players slumped on the turf in disbelief.

Kotaro Matsushima, hat-trick hero of Japan's 30-10 win over Russia in last week's tournament curtain raiser, posed an early threat on as Japan edged a cagey opening.

- Swarming pressure -

Fly-half Yu Tamura fluffed a makeable penalty, before Ireland took the lead after 13 minutes when the rampaging Ringrose rose brilliantly to snaffle Jack Carty's hanging kick into the corner.

Carty, stepping in for Ireland's talismanic playmaker Johnny Sexton, produced another moment of magic seven minutes later, dinking a clever chip that he managed to tip back to full-back Kearney to crash over.

Japan refused to buckle, however, and Tamura kept them in it with three clutch penalties.

Regular captain Michael Leitch's introduction after half an hour provided an instant impact, but Ireland survived to go into half-time up 12-9.

Tamura missed a three-pointer 14 minutes after the interval that would have tied the game.

But Japan's swarming pressure soon told, Fukuoka showing superb footwork to sneak over and give Japan the lead, triggering deafening cheers at Ecopa stadium.

Another Tamura penalty extended Japan's advantage to 19-12 with eight minutes remaining before Fukuoka effectively sealed the result with another tremendous burst that kept the ball in Irish territory until the final gong.

"Anyone that is utterly shocked hasn't seen how good they are," said Ireland captain Rory Best.

"We were on the wrong side of the penalty count, but Japan posed a lot of questions to us and unfortunately we couldn't come up with the right answers."

Ireland, who came into the tournament as the world's top-ranked team but have never won a World Cup knockout match, will look to bounce back against Russia.

Japan know they still have no margin for error with games against Samoa and Scotland to come.

"That was a massive effort," said game captain Lappies Labuschagne.

"It took a special effort but we had massive belief that we could do something different tonight."