Harzand completed the English-Irish Derby double when taking the Dubai Duty Free-sponsored Classic at the Curragh.
Dermot Weld's charge came out on top after a prolonged battle with his old rival Idaho - third at Epsom - from the Aidan O'Brien stable.
The leading pair pulled clear at the business end of the race, with Harzand, the 4-6 favourite, asserting close home in the hands of Pat Smullen.
Stellar Mass was third, with Ed Dunlop's British raider Red Verdon just behind in fourth after putting in good late work.
Smullen said: "He's such a great horse, he's got such courage and determination. Ryan (Moore, on Idaho) came with one good determined run, I needed this horse to dig deep for me and he did - how good is he?
"This matches Epsom, in front of our home crowd.
"The pressure was on and we pulled it off."
Smullen went on: "He's a dual Derby winner now. He's so courageous.
"You can give horses good rides, bad rides - but when they're as good as this lad and they're as determined as this horse it just makes it easy. There's no knowing where this horse will end up.
"He's so tough and he's not just tough, he's got pace. He's a very, very good horse and I'd love to think he'd be an Arc horse."
Weld said: "I knew Aidan would have his horse better than at Epsom so I knew our horse had to be better than at Epsom to win again.
"He ran a magnificent race. He showed the speed that he has today. People were saying he didn't have speed, well you saw it today.
"Those two horses really picked up and quickened. Idaho is a high-class horse, he's always been that and he was at his best today. It was a great day, a great ride - a special day. It was a proper horse race.
"He'll have a good break now and we'll train him for the Arc."
The son of Sea The Stars remained unchanged with RaceBets at 6-1 second-favourite for the Prix de l'Arc de Triomphe behind 5-1 market leader Postponed.
Coral cut Weld's star to 5-1 (from 7-1) for the King George VI And Queen Elizabeth Stakes, for which Postponed is the even-money favourite