Watch: 14/1 shot Iridessa gives Joseph O'Brien his first Group One win in Britain as a trainer
Iridessa was a surprise winner of the bet365 Fillies' Mile at Newmarket.
Joseph O'Brien's youngster made a big impression when making a winning debut at Killarney in July, before finishing fifth in the Debutante Stakes at the Curragh and third in a Listed race at Leopardstown.
She looked to have her work cut out stepped up to Group One level, but struck gold at 14-1 in the hands of Wayne Lordan.
Pretty Pollyanna, winner of the Duchess of Cambridge Stakes and Prix Morny during the summer, was bidding to bounce back from a slightly disappointing effort in the Cheveley Park a fortnight ago and looked the likeliest winner when cruising to the lead under Danny Tudhope.
However, tackling a mile for the first time, she gave best inside the final furlong and it was left for Iridessa and Aidan O'Brien's 5-2 favourite Hermosa to fight it out.
Iridessa quickened up best and was always doing enough to hold Hermosa at bay by a length and a half. Pretty Pollyanna was another three-quarters of a length back in third.
Joseph O'Brien, saddling his first Group One winner in Britain, said: "I couldn't say we expected her to win, but we always loved the filly and we expected her to run well.
"She did it well and Wayne gave her a great ride. She travelled well and I thought she quickened up well. We're delighted.
"She ran well in the Curragh in a very messy race and then got drawn almost widest of all at Leopardstown, so on her last two runs she never got a fair crack at it really. Today was a fairly-run race and she showed how good she is.
Betfair went 16-1 about the winner for the Qipco 1000 Guineas, with RaceBets going 20-1 about her chance in the Investec Oaks.
O'Brien added: "She's definitely well entitled to line up in it (1000 Guineas). Being by Ruler Of The World, she's probably more of an Oaks filly, but we'll get today out of the way and worry about that when the time comes.
"We'll enjoy today and give her a nice break through the winter and dream about next year."
Aidan O'Brien said of Hermosa: "She ran a lovely race and we were delighted. We will train her for the Classics.
"I think she is versatile enough. That was good to firm ground today and I would say she would handle soft ground.
"I'm delighted for Joseph and she (Iridessa) is home-bred."
Michael Bell was pleased with Pretty Pollyanna's effort in defeat.
He said: "That was a fantastic run, it was just a pity the Irish had to invade!
"She looked as though she got the trip and it opens up options for next year. It was her first time running beyond six furlongs, so it was a different style of racing, but she has run a very good race.
"It is early to say, but a (Guineas) trial would make sense as that would be over seven furlongs.
"I would say she would almost certainly run in a trial."