Moonstone landed Aidan O'Brien's third successive Darley Irish Oaks in the tightest of finishes with another of the trainer's horses, Ice Queen.
Colm O'Donoghue looked likely to steer 66-1 shot Ice Queen to victory but Johnny Murtagh's favourite got up in the last stride to win by a short head.
Moonstone had finished second at the Epsom Oaks behind Look Here, who was not in action at the Curragh.
French raider Gagnoa took third behind O'Brien's two horses.
O'Brien was claiming his seventh successive Irish classic win and it brought up his haul of wins in his home country's classics to 19.
Moonstone went into the race as a maiden and O'Brien heaped praise on Murtagh's "great ride".
"We'll maybe look at racing her in the Yorkshire Oaks now," added O'Brien.
"It's another great day but I'm just grateful to be involved with so many great people.
"It was a great race with two jockeys and two fillies giving it their all.
"It was a usual masterful ride from Johnny and Colm gave his filly a great ride too.
"Ice Queen wouldn't mind dropping back to 10 furlongs."
The runner-up is in the Nassau Stakes at Glorious Goodwood on 2 August.
O'Donoghue was later hit with a two-day careless riding ban by the stewards after they looked into an incident in the closing stages involving the front two and third home Gagnoa.
Darley Irish Oaks (The Curragh)
1. Moonstone (J Murtagh) 2-1 Fav
2. Ice Queens (C O'Donoghue) 66-1
3. Gagnoa (J A Heffernan) 4-1
14 ran
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