Seabass, who came close to giving Katie Walsh a famous Grand National triumph at Aintree in 2012, has died at the age of 14.
Trained by Walsh's father, Ted, Seabass was third behind Neptune Collonges and Sunyhillboy as Katie achieved the highest position for a female rider in the world's greatest steeplechase.
He was sent off the 11-2 favourite in 2013 but could not repeat those heroics, though he did complete the course in 13th place as Auroras Encore landed the spoils.
Pat Glynn, of his owners the Gunners Syndicate, said the horse died of a heart attack on Tuesday.
Glynn added: "He was a super horse for us and we have some great memories of him.
"He won seven races in a row at one stage before finishing third at Aintree.
"He had two serious injuries, too. First he had ligament trouble as a young horse, then he was injured again and came back and ran in the National.
"He kept coming back against all the odds. The vet said he was 80 per cent gone the last time he was injured but he came back and won more races."
Seabass won seven of his 27 starts, was placed eight times, and amassed over €293,000 in prize-money.
His last racecourse appearance was at Fairyhouse in February when he was last of the five finishers in a hunter chase.