The younger brother of Joseph O’Brien, the 16-year-old had only ridden his first winner last month but defied his inexperience with a fine ride on his father Aidan’s five-year-old, running in the colours of his mother, Anne-Marie.
Burdened with top-weight, O’Brien’s 7lb claim proved invaluable as Hidden Universe and Pat Smullen pressed hard through the final furlong.
El Salvador found plenty for pressure, however, and saw it out for a length success at 25-1.
Marchese Marconi, the mount of Joseph O’Brien, was third, with Asbury Boss fourth.
Aidan O’Brien said: “I’m delighted and it’s great for Donnacha. It’s a hard race to win. He’s a tall fellow and is doing his leaving certificate this year. It was a nice, patient ride as the start was a bit messy and he was a long way back.
“He was second in the Queen Alexandra Stakes at Royal Ascot this year, and is a tough horse.”
It was a fourth winner for Donncha O’Brien and a second Irish Cesarewitch for O’Brien after Bright Horizon 2010.
El Salvador was the final leg of a treble on the day for Ballydoyle, with Royal Navy Ship (6-1) coming out on top in what is likely to prove a very strong Irish Stallion Farms European Breeders Fund Maiden.
Although green when he hit the front, Seamie Heffernan’s mount kept on well, eventually having a length and a half in hand on another O’Brien runner, the 100-30 favourite Jacobean, who finished best of all down the centre of the track from the rear in the hands of Joseph O’Brien.
O’Brien, who had four runners in the race, said: “I’m delighted with that fella. He was well drawn, while Joseph’s horse was badly drawn.
“It comes to a time of the year when they have to start, and there wasn’t much between three of ours coming here.”
Together Forever had the best form on offer in the Staffordstown Stud Stakes and obliged with the minimum of fuss.
Sent off the 11-10 favourite and kept out of trouble on the outside of runners by Joseph O’Brien, the daughter of Galileo was wound up to strike two furlongs out and asserted the further she went.
Heffernan and Cocoon made it an O’Brien one-two, but never threatened the three-length winner.
O’Brien said: “She quickened up well, and Joseph feels she is more of a Guineas than an Oaks type. She has more speed than we were giving her credit for. Before she won her maiden at Gowran we were making too much use of her.”
Viztoria returned to winning ways with an impressive success in the Go Racing In Kildare Waterford Testimonial Stakes.
Eddie Lynam’s classy four-year-old remains lightly raced and was having her first run since finishing down the field in the Prix Maurice de Gheest at Deauville in August.
Dropping markedly in class to Listed level, the 15-8 favourite arrived on the scene full of running under Pat Smullen two out, quickly accounting for An Saighdiur, who had cut out the pace in company with Tylery Wonder.
She soon put the race to bed when asked, going three lengths clear of stablemate Gathering Power, who ran a good race in second.
Lynam said: “It’s great to have a good horse, but even better to have a good owner. They have been very patient, and that’s the first time in a long time that she’s had her ground. There has been no problem, we have just been waiting on the ground.
“She ran on very bad ground in France and ran poorly. We never found a reason for it, but that’s the only blip on her CV.
“We flew her over and she’d never been on a plane before and maybe that gave her a fright.
“She will go for the Champion Sprint (at Ascot) next Saturday, and is entitled to take her chance.”
RaceBets cut Viztoria to 10-1 from 14-1 for the Ascot Group Two.
The curtain came down on the season at the Curragh as Luca Cumani’s Newmarket raider Second Step claimed the Listed Finale Stakes.
It was a brave performance from Andrea Atzeni’s mount (13-2), who had looked booked for a place after being headed Chance To Dance, who was in turn pressed by Blue Hussar on this outside. But Atzeni conjured another burst from his partner, who got up to win by half a length Chance To Dance. The winner was running in the colours of Craig Bennett’s Merry Fox Stud.