Thistle Bird dispelled any stamina concerns as she came out on top in the Newbridge Silverware Pretty Polly Stakes at the Curragh.
Her previous seven wins had come at a shorter distance, but she stayed the mile and a quarter in no uncertain terms she took the Group One honours for trainer Roger Charlton and jockey George Baker.
Last year's winner Ambivalent tried to lead all the way but she was stalked by Venus De Milo, who went on early in the straight.
That move was covered by a confident Baker on Thistle Bird (4-1), who made eyecatching progress to lead a furlong out and win tidily by two and three-quarter lengths.
Just The Judge put in her best work in the closing stages to finish third, with Ambivalent fourth.
Charlton said: "I'm really delighted. There's not many things trainers get lucky at, but for a mare of that age to win a Group One is hugely satisfying.
"She travelled well and relaxed really well. She doesn't mind whether it's firm or heavy ground. She's tough and she's improving and it says a lot about keeping horses in training.
"She's had a few problems, but she's tough and sound and will be a great addition to the broodmare band.
"We were going to retire her after she finished second in a Group One (Nassau Stakes) last year, but the owner didn't have any other horses with me and I thought she could win good prizes.
"We delayed putting her in foal and she's done it now.
"It's hard to say (where next), but courses like Goodwood suit and she could go for the Nassau again.
"We'll pick and choose and take our time, there lots of nice races around Europe."
Baker, enjoying a fantastic season that has also included a Royal Ascot winner, said: "She travelled well and we got a lovely tow into the race.
"When she hit the front she was going through the motions, so it was a very good performance."