World No.2 and Masters champion McIlroy was staring down the barrel on Sunday (Monday AEST) when Lagergren posted 17-under in the clubhouse at The K Club, knowing only an eagle on the par-five 18th would keep his hopes alive -- and the Northern Irish superstar did not disappoint.
"Get in the hole," yelled the crowd as McIlroy played his shot and as they held their breath, the ball rolled in before they erupted as one with chants of "Rory! Rory!"
Both birdied the first two holes in the playoff but there was more drama on the third where McIlroy missed the fairway with his drive while Lagergren's ball bounced cruelly off the green and splashed into the water.
Ultimately, Lagergren could only manage par as McIlroy finished with a birdie, sending the crowd into raptures as he won the Irish Open again for the first time since 2016.
"I feel just so lucky that I get to do this in front of these people. The support has been absolutely amazing all week," said McIlroy, who started the final round four shots back of overnight leader Adrien Saddier.
"I thought it was going to be nice coming home with a Green Jacket, but this has exceeded all expectations.
"I love coming home. I love playing in this atmosphere. Moments like this, these are the things you're going to remember well after your career is over. This is a really special day."
For Lagergren, the loss was a tough one to swallow after taking what seemed a commanding clubhouse lead.
"I really thought I had that out there today. Obviously posting minus 17 going into the clubhouse, could have been enough," he said.
"I played really good in the play-off as well. Hit a solid five-iron down on the last play, but it got a horrendous bounce. It's a metre from being dead to the hole. So it's tough."
McIlroy's victory also provides perfect momentum ahead of the Ryder Cup where Team Europe will be defending their crown at Bethpage Black Course, Farmingdale, later this month.
Having completed the career grand slam in April, an elated McIlroy said: "2025's going to be one of the best, if not the best, of my career.
"But we're not finished yet, I've got a big week next week at the PGA Championship at Wentworth and then obviously everybody's looking forward to the Ryder Cup."
Elvis Smylie, the only Australian to make the cut at The K Club, finished tied for 37th after a final-round 73 left him at three under for the tournament.
