He Knows No Fear created history at Leopardstown when winning the Irish Stallion Farms EBF Maiden at 300-1.
Before his success, the longest-priced winner in Ireland was Killahara Castle at 200-1 in December 2017.
Trained by Luke Comer, He Knows No Fear was 12th of 14 on his debut at Limerick last month - at 250-1.
Ridden by Chris Hayes, the three-year-old was closer to the pace on this occasion, but a place still looked the best he could achieve when Jim Bolger's evens favourite Agitare shot clear.
However, when that one began to tire in front, He Knows No Fear sprouted wings and got up in the dying strides.
The longest-priced winner in Britain is 250-1 shot Equinoctial at Kelso, in November 1990.
Comer's assistant Jim Gorman said: "He got left half a furlong in Limerick - so we didn't really know much after it - and because some of our horses weren't in great form at the time, we backed off them.
"Going to Limerick we thought he was a real nice horse but he got left so far we couldn't get any kind of guide to him. It was great because it's one of Luke's own stallions (Mourayan) and his own mare. That gives him the greatest thrill, to breed his own."
He Knows No Fear was a welcome winner for the yard, whatever his price. Gorman added: "We had a few quid on him each-way in Limerick but he broke so badly and was left in the stalls that day so we didn't really know.
"All our horses have been running well in the last few weeks and knocking on the door without winning, so it's just great to get a winner. Today was a learning curve for us with him, so we'll see what the handicapper does and see where we go from there.
"He's a work in progress and is going to improve, because he was a bit green."
Despite his huge odds, bookmakers reported plenty of people had taken a chance on the winner. William Hill took 86 bets on He Knows No Fear with the biggest a E16.59 each-way returning a E5973.52 profit.
It was a similar story for Paddy Power. "Almost 100 plucky Paddy Power punters lived up to their horse's name and somehow managed to pinpoint the winner," said spokesman Paul Binfield. "These punting heroes have either been struck by divine inspiration or are extremely shrewd form judges."