Patsy Byrne was described as a "great man" and a "special person" by two of the trainers who provided the owner with his most recent successes on the racecourse.
The accomplished businessman will be mourned across the horse and greyhound racing worlds after his death, at the age of 64, was announced on Monday morning.
From Duagh, Co Kerry, Byrne split his time between Ireland and Surrey and his pink and blue colours were seen carried to victory only recently by White Star Line in the Kerry National at Listowel.
The horse's trainer, Dessie Hughes, said: "I can't believe it. It's totally out of the blue. As far as I know he got out of bed and collapsed.
"He was a great character and was at Listowel to see White Star Line win the Kerry National a couple of weeks ago, which was fantastic for him.
"He was a great man who loved his horses and his dogs and anything to do with sport really.
"He'll leave an enormous gap for everybody that had anything to do with him."
We have been hit with some really sad news today that our great friend Patsy Byrne has died suddenly. https://t.co/u4fqPLrOkn— Jonjo O'Neill Racing (@JonjoONeill) September 30, 2013
Byrne's other recent major victory on the track came in this season's Northumberland Plate at Newcastle with the Jonjo O'Neill-trained Tominator.
O'Neill tweeted: "We have been hit with some really sad news today, that our great friend Patsy Byrne has died suddenly.
"He was a very special person. Bridget (Byrne's wife) and her family are very much in our thoughts."
He founded the Byrne Group firm, which has been involved in the construction of buildings such as the Olympic Stadium, the Emirates Stadium, Stamford Bridge and the new Centre and Number One Court at Wimbledon, in 1969 with his brother Johnny.
An enthusiast of all Irish sports, he was involved with top dogs like Cool Performance and Ballinderry Ash as well as enjoying major success on the coursing scene.
Byrne helped bring together the largely Irish group of businessmen who bought Sunderland FC from Bob Murray in 2006.
SAFC chief executive Margaret Byrne said on their website: "Everyone at the club was very upset to hear the news of Patsy's death this morning. He was a true gentleman - genuinely kind, with a generous spirit and all of those who knew him will remember his quick wit and mischievous sense of humour.
"He always had a good word to say about everyone and remained a great support to me personally and to the football club. He will be greatly missed by so many people.
"Our thoughts and prayers are with Bridget and the Byrne family at this time."
Others to offer their sympathy were champion jockey Tony McCoy, who tweeted: "Sad news about Patsy Byrne, a great man & friend, big loss 2 the racing and greyhound world, thoughts with his wife Bridget & family''
Trainer Brendan Powell tweeted: "Sad to hear about the passing of Patsy Byrne. Rode winners for him back in the days of Les Kennard. A generous and lovely man. RIP."
Jockey Wayne Lordan remembered a good association he struck up with the Byrne-owned Tolpuddle back in 2004.
He said: "It's very sad news to hear and my thoughts are with his family.
"Tolpuddle won the Irish Lincolnshire and I won a couple of Listed races on him.
"Patsy was a true gentleman. He loved his racing and he loved his dogs."
Sean Bourke, a friend of Byrne who trained his Cool Performance to win the Irish Greyhound Derby in 2001, told At The Races: "He was the straightest person I've ever dealt with.
"He loved his dogs and I'd say he preferred a good dog to a good horse, anyway.
"He kept all the Irish going in England, if anyone had a friend looking for work, he'd always look after them and he was great to the Irish guys over there.
"But he never left his roots and he loved the banter in the local pub. He was just one in a million really."
One of his closest friends was Punchestown general manager Dick O'Sullivan and the racecourse tweeted: "R.I.P Patsy Byrne. He was a great friend to us here at Punchestown. Our thoughts are with his family."
He is survived by his wife Bridget, sons Michael and Sean, and daughter Siobhan.