Search For A Song gave trainer Dermot Weld an eighth success in the Comer Group International Irish St Leger at the Curragh.
Having only the fifth start of her career, and the only three-year-old in the race, the daughter of Galileo landed the spoils under an enterprising ride by Chris Hayes.
He decided the pace being set by Master Of Reality and Latrobe was not strong enough so he moved the Moyglare Stud-owned filly to the front fully seven furlongs from home.
Search For A Song (10/1) - a winner at York on her previous start - soon built up a handy lead and was not for stopping and she galloped past the line two and a quarter lengths clear of Kew Gardens.
The latter made late gains without threatening the winner, who is the first three-year-old filly to lift this prize since Petite Ile in 1989.
Southern France was a further length and a half away third.
Weld said: "It's a very special win on a very special and emotional day. We're having wonderful horse racing, but I'm very proud of our industry the way we are supporting Pat Smullen and cancer research.
"This has been a very special race for me over the years and it's the eighth time I've won it.
"She was running very free early on and he'd a hard call to make. I was happy he made the right call because I didn't want to see him spend the rest of the race trying to settle her.
"I know how well she stays so I wasn't too worried. There is a lot of stamina there as her brother is Falcon Eight.
"She was very fit and very well, and often horses that run that free will relax when they get to the front.
"Potentially she is a really high-class stayer. That was as good a St Leger as I've won when you look at the ratings of the horses in the race. The world is her oyster next year, and there is huge improvement potentially in this filly. She loves fast ground.
"She put up an excellent performance at York and has progressed throughout the year. I'd say that will be it for this year, and hopefully she will make into a lovely four-year-old."
