Sizing John, ridden by Robbie Power, pipped Djakadam in a close finish to the Gold Cup at Punchestown.
The seven-year-old odds-on favourite, trained by Jessica Harrington, won by a short head to add to successes in the Irish and Cheltenham Gold Cups.
Coneygree, winner of the big race at Cheltenham in 2015, led most of the way and finished third.
"He was never travelling the way he did at Cheltenham, but pure guts got him there," said Harrington.
Sizing John was the 9-10 favourite to land the feature on the second day of the Punchestown Festival.
He did so in the most dramatic style to become the first to complete the Leopardstown, Cheltenham and Punchestown treble
The Mark Bradstock-trained Coneygree, making just his third appearance since winning at Cheltenham two years ago, took the field along for much of the extended three-mile journey and he remained in front rounding the home turn.
Djakadam, ridden by Ruby Walsh, and Sizing John cruised up ominously, but Coneygree rallied valiantly to set up a grandstand finish - despite having made a mistake at the second-last.
There was little to choose between the three jumping the final fence, but eventual runner-up Djakadam and Sizing John asserted soon after and it was the latter who won the battle by a short head.
BBC horse racing correspondent Cornelius Lysaght
What a terrific horse race, run at a great tempo, and with three in the air together at the last before a finish that raised the roof.
The bare facts are that Sizing John narrowly beat Djakadam, both of which put in superb efforts, the winner not as scintillating as at Cheltenham, but that was understandable after a long season.
The win also continued the dream run being enjoyed by jockey Robbie Power.
But what about Coneygree? The 2015 Cheltenham star, racing for the first time since November, really showed his mettle, leading them a merry dance for most of the way, and had he not pecked at the second last he might well have won. A magnificent return.