The 49-year-old superpub owner had been reported missing from a Dublin hospital before his body was found.
Friends and former business associates say he had been under huge stress since the collapse of his empire in 2009.
Fellow publican Charlie Chawke described the news as a "huge shock", while restaurateur Jay Bourke said the hospitality sector was devastated by Mr O'Regan's death.
The married father-of-four is thought to have died of natural causes.
He was famous for establishing the Thomas Read group of fashionable superpubs across Dublin, catering for Ireland's newly affluent Celtic Tiger cubs.
They included Pravda, Ron Black's, The Bailey, Searson's, Lincoln's Inn, the Budda Bar in Blanchardstown and the 40 Foot in Dun Laoghaire.
Mr O'Regan sold the pub chain in 2003 in a €35m deal.
But the financial crisis of 2008 led to the collapse of his business empire with debts of €260m, with NAMA assuming control of his debts.
He was found dead near his car on the hard shoulder of the N11 dual carriageway at Newtownmountkennedy, Co Wicklow on Monday evening.
There was no evidence that he had been struck by a vehicle or had suffered any injuries.
He had been reported missing by his family and his body was found by gardai.
Mr O'Regan, who was from Clontarf but lived in Sandymount, is survived by his wife Adrienne, four sons and his brothers Declan and Paul.
Mr Chawke said Mr O'Regan was an "awful loss to the industry".
Mr Bourke said the redevelopment of Temple Bar "was Hugh's idea, it was his vision".
A friend of the dead man said Mr O'Regan found his "vilification" following the collapse of his business "very hurtful".
Last July, Mr O'Regan was restricted from acting as a company director for five years by the High Court.
A project that was to prove his undoing was the redevelopment of Kilternan Golf and Country Club in south Co Dublin. His plans to turn the hotel into a leisure and learning campus were scuppered by the banking crisis.
His lenders, Anglo Irish Bank and Irish Nationwide Building Society, later called in their loans.
See Dan White, P16