Irons, from Hawaii, was an entrant in the Rip Curl Pro Search world championship tour event currently being held in Puerto Rico, but he was forced to withdraw from the competition through illness.
Billabong spokesman Andrew Flitton told Australia's Surfing Life that the 32-year-old had been ill with dengue fever.
Flitton said Irons first flew from Puerto Rico to Miami, where he spent two days on an IV drip.
He then insisted on heading home to see his own doctor, but became sick and missed a connecting flight to Hawaii from Dallas, Texas, Surfing Life reported.
He checked into a Dallas hotel to rest. Hotel staff found Irons dead the next morning.
The Association of Surfing Professionals was expected to make a statement shortly.
Irons was world champion in 2002, 2003 and 2004, winning 19 world tour titles - his most recent in September at the Billabong Pro in Tahiti.
He leaves behind his pregnant wife Lyndie Dupuis who he married in 2007.
Seriously, how the hell did this happen??? RIP. He was ridiculously good too. Him and Bruce have that unique aggro style that separated them from the crowd.