Leona Maguire found only Jennifer Kupcho too good at the Meijer LPGA Classic, finishing in a tie for second after succumbing to the American on the second sudden death play-off hole at Blythefield Country Club in Michigan.
Maguire, who finished runner-up at the tournament last season, sizzled to a closing 65 to finish on 18 under, alongside Kupcho and Nelly Korda.
Korda's sudden death hopes ended with a par on the first play-off hole on 18, with Maguire and Kupcho both recording birdies.
The eventual winner missed a three-footer for eagle to wrap up the victory on the first sudden-death hole, but her moment arrived on the second time of asking.
Maguire's errant drive off the tee found the rough, but her sublime second shot had looked to retrieve the situation.
However, the Cavan woman three-putted for par as the ANA Inspiration champion Kupcho birdied to register a second success on the LPGA Tour.
Maguire sat seven shots off overnight leader Nelly Korda at the beginning of play but launched a birdie blitz from the off, gaining strokes at the 1st, 4th, 5th and 6th - giving one back on the 3rd - to reach the turn in 33 to get to 14-under.
Further birdies followed at 10, 13 and 14 as the leading contenders, Korda and Kupcho, laboured. Maguire finished off with a closing birdie on the par-5 18th to set a clubhouse target at 18-under par.
Kupcho, who briefly hit 19-under before a bogey on 16 knocked her back, could only make par on 18 after finding the bunker with her drive. After a so-so round, Korda birdied 18 to card a 72 and ensure a three-way playoff.
The tournament looked to be settled in Kupcho's favour after a phenomenal approach left her a 3ft putt for eagle on the first playoff hole, the 18th, while Maguire found the heart of the green.
However, after Maguire could only two-putt for birdie, a jittery Kupcho proceeded to miss her short putt for victory. Korda departed the fray following a par.
And so Maguire and Kupcho came up the 18th again to settle the matter, and it was the Irishwoman's turn to miss a three-footer, handing victory to the American after a titanic battle.
Lydia Ko of New Zealand took fourth as she finished one shot behind the play-off trio, with Lexi Thompson, Jessica Korda, Carlota Ciganda and Atthaya Thitikul sharing fifth on 16 under.