Luxembourg pipped Onesto as trainer Aidan O’Brien took a fourth consecutive Irish Champion Stakes win at Leopardstown.
It was also a fourth win for jockey Ryan Moore, who edged the 7-2 shot ahead in the final stages to claim the victory in a highly competitive field.
Stone Age set the early pace but faded as Luxembourg and Onesto moved through the field.
Pre-race favourite Vadeni (7-4) had to settle for third.
Luxembourg returned to racing last month with a win at the Curragh after a lengthy spell on the sidelines.
A muscular injury kept the three-year-old out of action for much of the season, but he now appears back to his best following the impressive win.
“It was unbelievable, Ryan gave him a brilliant ride,” O’Brien told RTE.
“The plan was to go to the Epsom Derby and the Irish Derby [earlier this year] but obviously that went out the window.
“We were very lucky to get a run in to him at the Curragh. We knew he would be [missing] 20% or 30% which is an awful lot. But when he did what he did, we knew when he came back here that he had a chance.”
