Derby third Los Angeles went two places better at the Curragh June 30 as he landed the Irish equivalent under Ryan Moore.
The Aidan O'Brien-trained colt had been beaten three and a quarter lengths behind runner-up Ambiente Friendly at Epsom four weeks ago.
However, he reversed that form this time, giving O'Brien a record-extending 16th victory in the Irish Derby.
Moore sent Los Angeles to the front early in the straight and his mount was soon challenged by his Epsom conqueror.
He had enough to get the better of him this time and held off the strong-finishing Sunway by three-quarters of a length, with Ambiente Friendly another half-length back in third.
O'Brien said after the race: "We think he'll get better. He's a big hardy horse. He looks like he is a baby but he was a group 1 winner at two as well and he keeps progressing. We are really looking forward to him.
"Ryan said that they went hard and that he ended up getting there a little bit earlier than he wanted. He was very impressed with him and I think Ryan was a bit surprised with the class that he showed.
"We thought coming here that he was a horse who was going to get a mile and a half and turn into a Leger horse, but listening to Ryan he's a very classy horse too. That leaves a lot of options."
The winner is 3-1 favorite (from 4) for the St Leger with Paddy Power, who quote him at 7-1 (from 10) for the King George VI and Queen Elizabeth Stakes.
Trainer David Menuisier was pleased with Sunway and said: "That's the third Classic I've lost by less than a length this season so I'm used to it now!
"I'm delighted with the horse, we've always thought the world of him. People lost faith in the horse and today he vindicated himself and showed that he's a really classy act."
Trainer James Fanshawe felt Ambiente Friendly was beaten for lack of stamina and said: "He traveled really well to the two pole and just got outstayed in the last furlong.
"The winner and the second are both very good horses. I still think we'll probably be dropping back in trip next time."
Matsuri (GB) finished a close fourth after being stepped up in class from success in a novice event at Leicester by trainer Roger Varian, who said: "He's run a great race, he confirmed he's a very good horse. It's obviously pleasing to see him hold his own."
He did not get an entirely clear passage and Varian said: "I don't think we got quite the room to run when perhaps we needed it, but that's racing.
"He was beaten maybe a length in total, having had a slightly troubled passage in the home straight, so maybe we might think what could have been. But he's a good horse and he showed that today."
