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.