Splaine, competing in the event for the first time, won a five-horse jump-off with the only clear round to deny Britain's Michael Whitaker a second successive victory on Midnight Madness.
Splaine, who became the first Irishman to win the event since Tommy Wade on the famous Dundrum in 1963, described the win as the best of his career and said: "It's something I've always dreamt about riding in. It was a real honour and privilege to win and even to be in the line-up.
"This competition still has a lot of prestige attached to it - just ask any rider around the world. I've dreamt of this since I was a boy."
Splaine, who had a winner on the opening day with Blue Grass, earned the right to go last in the jump-off, having recorded the fastest clear in the opening round. After all the others faulted, Splaine decided to concentrate on going clear, regardless of the time it took him.
The gamble paid off as he went clear in 66.22sec, over 16sec slower than Whitaker.
A prize of pounds 5,000 for winning here may not compare well with the pounds 35,000 on offer in the Luxembourg Grand Prix today. While that prize money has lured away such riders as John Whitaker and Nick Skelton, Michael Whitaker bucked the trend by flying back overnight from the Luxembourg International to ride last year's winner, Everest Midnight Madness, in the event he has won four times before.
The pair came very close to regaining that magnificent Gold Cup and Whitaker, clearly disappointed, blamed himself for the single mistake - hitting the first part of the fifth fence - that cost him his fifth King George title after taking only 49.52sec to complete the round. It was a fast enough time to make him the favourite to win, despite the four faults, until the Irishman took his chance and produced the beautifully judged fluent clear round.
The timing order rule did no favours for Whitaker, who said: "The draw put me second and it didn't help. I hadn't realised the second round would be made on first-round times."
"I should still have won and I've no regrets about coming back for this class,"
William Funnell and Comex tried valiantly to peg back Whitaker after hitting the big upright second fence but failed by just one second to finish in third place.Reuse content