The Hobbit star is part-owner of eight-year-old Riverside Theatre, who was ridden by jockey Barry Geraghty to a memorable win at the prestigious Cheltenham Festival.
Nesbitt told reporters, ""Forget the Oscars, I've done it all now. I'm rarely lost for words but I'm very emotional now.
""It's pasted into the album of my memory. I'll die a happy man. It was a glorious celebration of everything that's good about sport, not just horse racing: the courage of the horse, the ride of Barry Geraghty.
""The horse was beaten at the last and then Barry asked him for a bit more. He was brave and he was strong. We knew he would stay and we knew he would jump but to have had that brilliant battle, I cannot believe it. It's incredible.""
The actor scooped a trophy as well as $416,000 (£260,000) in prize money.