Diamondsandrubies denied Legatissimo in a driving finish to the Sea The Stars Pretty Polly Stakes at the Curragh.
The Aidan O'Brien-trained filly made all under Seamie Heffernan and just held the challenge of the Qipco 1000 Guineas winner to claim the Group One spoils.
Diamondsandrubies (8-1) was reversing Investec Oaks form with Legatissimo, having finished two places behind that filly at Epsom when fourth to Qualify.
There was just a short head between the pair, with the James Fanshawe-trained Ribbons close behind in third under Frankie Dettori.
Heffernan said: "I rode her in Epsom and it didn't really swing my way. Sometimes Epsom finishes a horse and sometimes the good horses thrive on it. She's obviously good.
"We thought she'd stay and I jumped and she landed in front and I was happy enough to go along with it. I knew they (the challengers) were coming. I knew where the line was.
"I was a bit unlucky at Epsom. Aidan trains them to progress from run to run and today she was at her best, so hopefully there will be more improvement."
Diamondsandrubies, who had also impressed when winning the Cheshire Oaks, will now go for the Darley Irish Oaks on July 18.
O'Brien said of his filly, who had been badly hampered a furlong and a half out at Epsom: "She was very unlucky at Epsom and was probably the most inconvenienced by the whole thing. She was always going to come here and we said we'd let Qualify take her chance in the Derby.
"We'd like to have got a lead but that's not the way it worked out, and Seamie gave her a great ride. Emmet McNamara rides her out every day at home and does a great job.
"We know the dam well and I'm delighted for Richard and Roisin (Henry, part-owners with Sue Magnier).
"We'll ride her a bit more patiently over a mile and a half, and Qualify and Words will also run in the Irish Oaks."