The son of Lawman was heavily supported for last season's 2000 Guineas amid reports of sparkling workouts on the Manton gallops, but injury forced him to miss his intended prep run in the Craven Stakes at Newmarket and the Rowley Mile Classic itself.
He suffered a troubled passage when he did eventually make his three-year-old debut in the Prix du Jockey Club at Chantilly, before enjoying his day in the sun when grabbing Group One glory under Kieren Fallon at Royal Ascot.
Most Improved failed to make a significant impact on his final three starts for Meehan and was last seen finishing last of eight behind O'Brien's star miler Excelebration in the Queen Elizabeth II Stakes on Champions Day at Ascot.
A statement on Meehan's website read: "Most Improved, the smart miler who provided the Brian Meehan stable with one of the highlights of the 2012 season when winning the Group One St James's Palace Stakes at Royal Ascot, has been sold to go into training with Aidan O'Brien at Ballydoyle.
"The Lawman colt has been bought by the Coolmore partnership to race on as a four-year-old. All of us at Manton wish the new owners all the best for the future with this high-class performer who has been such an excellent servant for us and his owner, Mr Iraj Parvizi, over the past two seasons."