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."
Joseph O'Brien said: "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."
He added: "He (Donnacha O'Brien) had a perfect position all the way, and committed him at the perfect time. The horse is tough and stays well."