Thundering Nights just edged out Santa Barbara in a tremendous finish to the Alwasmiyah Pretty Polly Stakes at the Curragh.
In action in America at Belmont just 23 days ago when beaten a nose in a Grade Two by Mean Mary, Thundering Nights was taking her form to a new level.
Santa Barbara, meanwhile, was running in her third Group One of the season and having been the beaten favourite in both the 1000 Guineas and Oaks she displayed the talent that had Aidan O'Brien speaking so highly of her in the spring.
The improving Epona Plays set out to make the running and was only headed on entering the final furlong.
It was there that Shane Crosse decided to commit on Thundering Nights (16-5) and she went over a length clear, but Ryan Moore had been biding his time on Santa Barbara and she gradually hit top gear, drawing level with Thundering Nights.
The two were locked together inside the final 100 yards - but in the final few strides Joseph O'Brien's four-year-old, who was conceding 12lb to her younger rival, just began to pull away and got the verdict by a neck.
The favourite Cayenne Pepper was two and a half lengths away in third, as Crosse claimed the second Group One success of his career following the victory of Pretty Gorgeous in the Fillies' Mile last season.
"She's a great mare, she's very tough and every time you ask her a question she rises to the occasion," said O'Brien.
"She's never run a bad race really and I'm delighted for Shapoor Mistry, the owner. I'm very proud of her.
"She always has a bit of a look around when she goes to the front and she did that again today, but when the other one came to her she really fought back.
"Broome just nabbed her here one day on the line, but she had an opportunity to fight back today and she did that.
"In America she would have won in another stride, the tight track caught her out a little bit as it was her first time around the bends over there.
"She had to spend a week after the race over there so she's only been back a couple of weeks. She trained there for a week, then shipped straight back and came straight here. She has a great constitution.
"She has options back in America, like the Beverly D., and she also has the likes of the Nassau and Matron.
"She's a Group One winner now so she's a very exciting broodmare prospect no matter what happens."
O'Brien added: "Shane deserved that because over the last couple of days a few things didn't go well for him.
"That's the way it goes in racing and you just have to remain confident and keep doing what you are doing. That's what he's done and he was rewarded."