Aidan O’Brien hailed Giant’s Causeway as ‘incredible, brave and tough’ as he saluted the life of the champion dubbed the ‘Iron Horse’, who has died aged 21.

So named for the battling qualities which carried him to five Group 1 wins during a remarkable three-year-old campaign, Giant's Causeway, later a leading sire in North America, died on Monday night at Ashford Stud, his Kentucky birthplace.

O'Brien, who trained the son of Storm Cat through a 13-race career in which he never finished out of the first two and raked in more than £2 million in prize-money, said: "He was an incredible horse, really.

"He was brave and consistent and he just loved a battle. He's a great sire of broodmares and he puts that toughness that he had into all of his progeny.

"He was a true champion and such a solid horse who raced at the top level and gave it his all each time. He really was incredible."

Legendary season

Unbeaten as a two-year-old, Giant's Causeway finished second in both the 2,000 Guineas and Irish 2,000 Guineas before a remarkable run of five straight Group 1 wins that were the bedrock of his legendary status.

It was not just the wins but the manner of them that helped sear his status into racing’s collective consciousness.

In a glorious summer, victory in the St James’s Palace Stakes at Royal Ascot under Mick Kinane was followed by success in the Eclipse, when ridden for the only time by George Duffield.

In both Giant’s Causeway refused to be beaten as he was headed and then battled back to beat Valentino a head at Ascot and Kalanisi by the same margin in the Eclipse at Sandown.

Victory in the Sussex Stakes at Glorious Goodwood by three-quarters of a length was his most emphatic of the five and foreshadowed further successes in the International at York and the Irish Champion Stakes at Leopardstown.

His second in the QEII at Ascot heralded a trip to the Breeders’ Cup Classic at Churchill Downs where he lost by a neck to Tiznow.

Named Cartier Horse Of The Year in 2000, he retired to stud at the end of his three-year-old season, standing with Coolmore in Ireland before transferring to their Australian and then American units.

Footstepsinthesand and Ghanaati, winners of the 2,000 Guineas and 1,000 Guineas, were among his progeny and he was leading sire in North America in 2009, 2010 and 2012.

Once-in-a-lifetime horse

Coolmore America manager Dermot Ryan said: "To foal, raise and race a horse of Giant's Causeway's calibre is every owner/breeder's dream, so for him to stand as a stallion at his birthplace, and go on to be a three-time champion sire is nothing short of sensational.

"Giant's Causeway was a once-in-a-lifetime horse, and I would like to thank the owners, the Magnier and Tabor families, for entrusting us with his stud career.

"I would also like to extend my gratitude to all of the stallion handlers at Ashford past and present for providing the highest level of care for Giant, in particular Blaise Benjamin and Richard Barry. He will be a huge loss not just to us, but to the industry as a whole."