His death is being investigated as a possible methadone overdose, according to Honolulu's Star Advertiser.
Irons, 32, had been suffering from dengue fever, a tropical illness whose symptoms include severe headaches and joint pain, according to his family. Methadone, a powerful pain-management drug, was reportedly found in his hotel room at the Dallas/Fort Worth airport.
An autopsy was scheduled for Wednesday. Results of toxicology tests to determine the exact cause of death could take 60-90 days.
Irons, the world champion from 2002 to 2004, had just returned to competitive surfing after a year off. He had been on his way home to Hawaii on Tuesday after withdrawing from a contest in Puerto Rico.
The surfing world reacted with shock to the news of his death. "The kind of surfing he did was so creative and so different from any other surfer," pro surfer Evan Valiere told the Star Advertiser. "He was real fluid, but real powerful at the same time. When he was on, I don't think anybody could match him."
Valiere said news of Irons's death spread quickly Tuesday on Kauai, where he grew up. "Nobody could believe it," he said. "You expect somebody like that to live forever."