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.
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 into 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 halftime 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.
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, whose target is a first-ever place in the last eight, know they still have no margin for error with games against Samoa and Scotland to come.
Japan stuns Ireland 19-12 in huge World Cup upset
Kenki Fukuoka struck the killer blow as hosts Japan stunned Ireland 19-12 on Saturday to take a giant step towards the Rugby World Cup quarter-finals.
The replacement winger wriggled over on the hour mark to score the winning try of a frenetic Pool A clash as the Brave Blossoms pulled off a breath-taking upset.
Japan's Brave Blossoms can now add the "Shizuoka Shock" to the "Brighton Miracle" after toppling South Africa at the 2015 tournament.
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.
Fukuoka made the difference, wriggling over on the overlap to complete a stunning team try and put the home side in front - where they stayed, 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.
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 into 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 halftime 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.
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, whose target is a first-ever place in the last eight, know they still have no margin for error with games against Samoa and Scotland to come.