He had gone to the Derby unbeaten and on the back of an eight-length romp in the Chester Vase.
O'Brien feels the time is right to get Sir Dragonet back on the track and drop him down in trip to a mile and a quarter for the first time.
The Ballydoyle handler is using this Group Three contest as a stepping-stone for Sir Dragonet to bigger targets.
"I'm very happy with Sir Dragonet," said O'Brien.
"Mentally and physically he is very good and the plan is to run in the Royal Whip and then on for either the Irish Champion Stakes or Prix Niel."
Sir Dragonet was among seven declared, with rivals including Buckhurst, trained by Joseph O'Brien and winner of the Group Three International Stakes over the course and distance seven weeks ago.
Jessica Harrington is hoping Leo De Fury has better luck than he endured at Goodwood, where he did not enjoy the best of runs when unplaced behind Nayef Road in the Gordon Stakes. The Australia colt had won his only two previous starts in promising fashion.
"He had a bit of problem when he knocked around a bit in Goodwood," said Harrington.
"He came out of the race fine and we're taking the next step.
"With Aidan's one in there, a very highly-rated colt in Sir Dragonet, we'll have to see what we do."
Jim Bolger saddles Irish Derby fifth Guaranteed, while Luke Comer's Zabriskie and Dermot Weld's Shelir complete the sextet.
Aidan and Joseph O'Brien are responsible for all six runners in the other Group Three feature, the Comer Group International Irish St Leger Trial Stakes.
Master Of Reality, third to Stradivarius in the Gold Cup at Royal Ascot, heads from O'Brien junior's stable, backed up by Downdraft, Cimeara and Eminent Authority.
O'Brien senior supplies dual Classic hero Capri and Southern France, who took fourth place behind Stradivarius in the Goodwood Cup on his latest outing.