Smullen announced his retirement from racing in May 2019 after completing treatment for pancreatic cancer, and won widespread praise for his stoic attitude and humility in the face of illness.
He won the Derby at Epsom in 2016 on Harzand, trained by Dermot Weld.
Smullen also landed the Irish Derby with Grey Swallow in 2004 and Harzand in 2016.
During his career, he also won the Irish 1,000 Guineas twice (Nightime and Bethrah), the Irish Oaks aboard Covert Love and the Irish St Leger four years running on Vinnie Roe.
As well as Harzand's Epsom triumph, his major wins for Weld in Britain included the 2,000 Guineas on Refuse To Bend, the Gold Cup at Royal Ascot with Rite Of Passage, and the Champion Stakes on Fascinating Rock.
Smullen's death was announced exactly a year after a legends race he organised, won by AP McCoy, helped raise more than 2.5m euros (£2.3m) for pancreatic cancer trials and research.
"Everyone has given so much support to the amazing man that Pat Smullen is - a special person and great friend to us all," said McCoy after victory at the Curragh.
Tributes were paid by jockeys after they learned of his death.
"Pat Smullen was the ultimate professional," said Ross Coakley.
"Unassuming, competitive and determined, he set the bar for all riders coming through. Feel privileged to have had the opportunity to have shared the weigh room with him. Thoughts and prayers are with his family #RIPPat #PJS"
Smullen leaves behind his wife, Frances, and their three children Hannah, Paddy and Sarah.
Horse Racing Ireland said that Smullen, who was diagnosed with pancreatic cancer in March 2018, died at St Vincent's Hospital in Dublin on Tuesday evening.
His initial treatment had been positive, but he suffered a relapse and was forced to abandon plans to ride in the charity race a year ago.
HRI chief executive Brian Kavanagh said: "Pat Smullen was one of Irish racing's brightest stars, a nine-time champion, but his achievements in the saddle pale in comparison to his qualities out of it. An inspiration to us all, his legacy is large."
A British Horseracing Authority statement said his death was "devastating" news.
"He will forever be remembered as one of the greats - not only at home in Ireland, but here in Britain and across the racing world," it said.
The President of Ireland, Michael D Higgins, expressed his "deep sadness" and said the jockey's "remarkable performances at home and abroad brought joy to so many. "
