He died on Wednesday despite the efforts of medics to save him.

The Florida-based rider, who moved to the US to compete in 1999, represented Ireland in the Nations Cup on several occasions and was seen as having a huge future in the sport.

Representatives from the Kentucky National Horse Show are to plant a tree at the horse park in his memory.

Prayer services have also been held in Lexington, Kentucky as Grimes’ family make funeral arrangements.

President of the Equestrian Federation of Ireland Charles Powell offered sympathies to the family.

“Our sincerest condolences go to the family. Our thoughts and prayers are with them on this sad occasion,” Mr Powell said.

“Niall was a highly-respected horseman and a noted participant in his chosen discipline of show jumping.”

Grimes was regarded as brave, generous and popular by his peers.

A spokesman for the EFI recalled two incidents were Grimes put himself second for the benefit of the team.

In March 2006, he rode for Ireland despite having fractured a bone in his leg in a fall a few weeks earlier. He had his leg strapped but set off over the course in typically determined and proud fashion to represent his country.

And earlier this year, on his last appearance in the Nations Cup, he broke a leg in a fall but offered his prized mount ’Mr President’ to another rider, Conor Swail, in order to secure maximum points for the team.

“It was an act of unusual generosity as riders can be quite protective of their horses. He was well regarded. He was that sort of guy,” the spokesman said.

Grimes was attended by medics within seconds of collapsing. He was revived twice in the first 45 minutes and treated at St Joseph’s Hospital, Lexington which houses one of America’s top cardiac care units.

He never fully regained consciousness and died on Wednesday afternoon with family, who had travelled from Ireland, and friends at his bedside.