Latrobe, trained by Joseph O'Brien and ridden by his brother, Donnacha, won a thrilling race for the Dubai Duty Free Irish Derby at the Curragh.
The son of Camelot, who took this prize in 2012 in the hands of Joseph O'Brien, battled on superbly to deny Rostropovich and Saxon Warrior, both trained by their father, Aidan.
Latrobe was the last in the stalls and after a messy first few furlongs, Rostropovich took the lead under Padraig Beggy.
Saxon Warrior, the 2000 Guineas winner who was fourth in the Investec Derby at Epsom, looked perfectly poised but he could never quite get to the front.
Latrobe (14-1) hit the front with two furlongs to run, but was made to fight all the way to hold the game Rostropovich, who went down by half a length, with Saxon Warrior (even-money favourite) another neck away in third.
Donnacha O'Brien - winning his third Classic of the season after Saxon Warrior at Newmarket and Forever Together in the Oaks at Epsom - said: "I didn't plan to be that forward, but the pace was slow. I held my position and he stayed really well to the line.
"It's pretty incredible and it's not really sinking in yet. It's a special moment.
"We've always thought he was a really good horse and he's never run a bad race, but we just thought a few times that he'd run better.
"He's shown today that he's very good."
Appropriately, Latrobe is a son of Camelot and was also winning in the Lloyd Williams colours that Joseph O'Brien sent out to victory in the Melbourne Cup through Rekindling.
The trainer said: "We had intended on taking our time a bit, but Donnacha let him go forward as there wasn't much pace on and that was the winning and losing of the race.
"He had a perfect position all the way, and committed him at the perfect time. The horse is tough and stays well.
"I have to thank everyone in the yard, especially Joey Fallon, who rides him out, and Lloyd and Nick (Williams) have been so good to me. I'm so grateful and lucky.
"We always loved him and bought him as a yearling in Newmarket. From day one we loved him and he never really let us down."
Looking to future plans for Latrobe, O'Brien, who also won the Irish Gold Cup over jumps with Edwulf in February, added: "I'd say that's going to be his perfect trip.
"I wouldn't have minded if the ground was a bit jucier, but obviously he doesn't mind quick ground. We weren't too worried about quick ground and I'd say he's versatile enough.
"I really don't know where he will go next. We'll have to see how he comes out of this and then decide."
He went on: "Everything myself and Donnacha have learned has been from dad and mum (Anne-Marie O'Brien), and we are so lucky to have been brought up in Ballydoyle.
"I don't think we have ever actually fallen out. We've had disagreements, but we've never fallen out. Myself, Donnacha, Sarah and Ana (sisters) all get on very well together.
"We are all very competitive with each other and against each other, but I'm as happy when they win as they would be when I win. Life is too short to be falling out over trivial things like sport.
"There are so many variables that can happen during a race, a jockey has to be able to make up his own mind. There is no doubt that Donnacha is riding well and it was his move that won the race.
"My association with Lloyd came through Order Of St George, who I won on a few times. When I started training he expressed an interest in getting involved with a yearling, and the horse was called Arcada who was quite useful and was sold on to Hong Kong afterwards.
"David Wachman then retired and (Williams) replaced him with Rekindling. We have been very lucky, so long may it last."
Aidan O'Brien spoke of his pride at the achievement of his sons.
He said: "It's an unbelievably special day, you couldn't dream of it. It would have been a sickener for us to chin the two lads, as I know how hard they work and we know how hard it is and how tough the competition is. I'm over the moon.
"We beat the lads most of the time, but you like to see everyone getting a chance. The two lads work so hard day in, day out.
"We know how competitive it is. Irish racing is every day. It's tough. Obviously we try to win every race, everyone does.
"We're so delighted for them. We are so grateful to everyone for supporting them."
He added of Saxon Warrior: "We were delighted with the run. We always thought he would be going back to a mile and a quarter after today and that's where he will be going.
"He travelled very strong early and was plenty strong. The plan was always to go back (in distance).
"We knew the lads knew their horse stayed. They got a nice lead and had him in a nice position, so it was going to be tough all the way up that straight at the Curragh.
"We'll probably give him a little rest and obviously the Juddmonte (International, at York) and the Irish Champion are the races we've been looking at with him."