Sir Dragonet could only finish second on his comeback behind impressive winner Numerian in the Devoy Stakes at Naas.
Sent off favourite for the Derby at Epsom in June having impressed hugely in the Chester Vase, he was beaten less than a length behind stablemate Anthony Van Dyck in the blue riband, after being supplemented following his effort on the Roodee.
He went on to finish fourth in the St Leger behind Logician and despite dropping half a mile in trip for his return to action, he was still expected to get his season off to a winning start as the 8-15 favourite.
Everything appeared to be going to plan for Seamie Heffernan with two furlongs to run as he moved into contention, but while Declan McDonogh was briefly short of room on Numerian, once in the clear
Joseph O'Brien's four-year-old shot past went on to win by two and three-quarter lengths.
O'Brien's representative Brendan Powell said of the 15-2 winner: "It was a nice performance, he did it well.
"Declan said he was a little bit on it early on, but they quickened up a long way out and he just sat and waited.
"He stays well and he's tough. He seems to have improved again and he's a fine big horse.
"I wouldn't have said it was his ideal ground, but he galloped away and seems to go on most ground.
"It was a nice performance first time out and you'd think he can only improve from it. I think he'll improve in every way and hopefully he could turn into a nice group horse.
"He looks the finished article this year compared to last year."
