He Knows No Fear became the longest-priced winner in horse racing history when he secured a 300-1 victory at Leopardstown.
Owned and trained by Luke Comer, and ridden by jockey Chris Hayes, the horse took victory in the Irish Stallion Farms EBF Maiden on Thursday.
The previous biggest winning SP was 250-1 by Equinocital at Kelso in 1990.
He Knows No Fear was making his second start after finishing 12th of 14 at 250-1 in Limerick last month.
The three-year-old colt made rapid late headway to secure victory by a head from the 98-rated Agitare.
In 1990, Equinoctial was successful in the Grants Whisky Novices' Handicap Hurdle at Kelso. Beaten by 62 lengths prior to recording his huge upset win, he had also fallen once and been pulled up twice.
"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," said Comer's assistant Jim Gorman.
"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."