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.
Her trainer said afterwards: “Yeah she did it nicely, we always thought the world of her and she's beginning to show it now.
“It was a messy race at the Curragh and at Leopardstown we were widest of all on the inside track so this was a huge step up.
“She was always going to get the trip well, having already won over a mile, and Wayne gave her a fantastic ride.
“It's very windy out there and the draw enabled her to get some cover, which really helped her settle.
“I guess she is probably more of an Oaks filly than a Guineas one, given her pedigree.”
Paddy Power make the winner a 20-1 chance for next year's Investec Oaks.