Sizing John, ridden by Robbie Power, has won the Cheltenham Gold Cup ahead of Minella Rocco and Native River.
The 7/1 winner, trained by Jessica Harrington, surged to victory by two and three-quarter lengths.
It means the seven-year-old has now completed the English-Irish Gold Cup double.
The favourite, Djakadam, ran out of steam, while Lizzie Kelly only reached the second fence. For the second successive year, Cue Card fell at the third-last.
Power won the Grand National on Silver Birch in 2007.
Commenting on his Gold Cup victory, he said: "Unbelievable. Jessica Harrington is a genius.
"I always said when this horse stepped up in trip he'd win. It's 10 years ago I won the National, I'm 35 now and appreciate it a bit more."
He added: "I had a lot of confidence in this horse, ever since the first day I rode him behind Douvan.
"I ended up jumping to the front at the second-last and I didn't want to do that until the last, but I was confident he'd stay up the hill."
Ms Harrington, who is based in County Kildare, had never had a runner in the race before.
"I can't believe it," she said. "It was amazing how he jumped and travelled.
"Robert said he wasn't going to go down the inside, but he did. It's absolutely fantastic, my first runner in a Gold Cup.
"This is the jewel in the crown, I've been watching and listening for as long as I can remember."
She added that Sizing John was "only seven so I've got to keep him right and mind him".
Owner Alan Potts commented: "It's unreal, it's my dream and it's come true."