Nicknamed the "Iron Horse" on account of his toughness and constitution, Giant's Causeway was hailed by his trainer Aidan O'Brien as a "true champion".
His winning sequence was broken in the Queen Elizabeth II Stakes at Ascot on 23 September, when he was beaten half a length by Observatory.
On his final racecourse appearance, Giant's Causeway was sent to the United States to contest the Breeders' Cup Classic at Churchill Downs in which he raced on dirt for the first time.
Starting at odds of 7.6/1, he was beaten only a neck into second place by the American colt Tiznow.
He stood one season there before relocating to Coolmore's United States branch Ashford Stud in Versailles, Kentucky.