Padraig Harrington beat Vijay Singh at the second play-off hole to claim his first PGA Tour title in the Honda Classic in Florida.
Joe Ogilvie also reached the play-off after the trio carded four-round totals of 14 under-par, but he was eliminated at the first hole.
Harrington had not been in contention until he produced a course-record equalling nine-under-par round of 63.
Singh, who lost his world number one ranking last week, turned in a 64.
Ireland's Harrington climbed from eighth to sixth in the world when the latest golf rankings were announced on Monday.
The 33-year-old has had to settle for second place 26 times since turning professional in 1995.
"When you have a win that maybe somebody gives it to you a little bit, it's nice when somebody does that, considering how many times I've been close and it's been taken away from me," said Harrington, a winner of nine European titles.
"I've come a long way to do it, I've got to say.
"I'm sure when I turned pro, nobody would have predicted how far I've gone, and it's days like that that makes it all worthwhile."
In an amazing fightback, he bagged 11 birdies in his final round to leap from joint 21st to being in the play-off.
Ogilvie was eliminated from the play-off at the first hole, and Singh had a great chance to win the title but saw the ball curl off to the right, handing Harrington a lifeline.
At the second hole, the pair's shots almost matched one another as far as the green, but Singh's miss of a relatively straightforward four-foot putt handed Harrington victory.
"I was mentally prepared to go back to the tee - it was a surprise"
Afterwards, he admitted he had been amazed by Singh's decisive error.
"I thought it was a gimme. It was a shock," said the world number eight.
"I was mentally prepared to go back to the tee. It was a surprise - it took me a couple of seconds for it to sink in because I was really focused."
He also said it was a matter of great pride to be the first Irish winner of a USPGA event.
"It means a lot now I've done it. Now it's done, they can't take that way from me," said Harrington, who will now hope to emulate last year's Classic winner Todd Hamilton, who followed it up with victory at the Open.
Singh was clearly disappointed, but was not dwelling on his misfortune.
"I hit the putt a little too hard, you can't miss putts like that in a play-off," he said.
"I played well, I struck the ball well and could have made one or two more putts out there, but I made the ones that counted."