Bellshill saw off the challenge of stable companion Djakadam to give trainer Willie Mullins his fourth victory in the Punchestown Gold Cup.
Mullins saddled four of the 12 runners, and it was Djakadam (5-1) who led for most of the race.
But 4-1 shot Bellshill - ridden by David Mullins - tracked him into the straight and came out on top in a thrilling duel after the final fence.
Road To Respect, the 7-2 favourite, passed the post in third.
Bellshill had finished a weary fifth in the Irish Grand National just over three weeks ago.
"This is brilliant," said the winning jockey. "To get one for Willie this week means a lot.
"The old horse does everything so perfect, it's just a shame that when he comes down to the last he gets a bit idle.
"I thought I'd have won the Irish Grand National on him, but that's racing and finally I got a big one on him."
Horse racing correspondent Cornelius Lysaght
Talk about zero to hero - although Paul Townend, riding Killultagh Vic, didn't win the Gold Cup, his three successes 24 hours after snatching defeat from victory with a manoeuvre in the Champion Novices Chase that cost him a 21-day ban went some way to erase the memory of that bizarre incident for which he's apologised.
The wins, two of six during the day for Willie Mullins, also wiped out Gordon Elliott's lead in the trainer's title race - decided by prize money - with three days of the season to go. But it remains close.