An official cause of death has yet to be released but a close friend denied speculation that Irons, 32, had a drug problem.
"The guess is that he probably took a sleeping pill to get a good night's sleep and get on the plane the next morning," Randy Rarick told "Good Morning America."
But the police report noted that generic versions of the prescription drugs Ambien and Xanax were found in the Dallas hotel room where Irons was found dead Tuesday. And the Honolulu Star-Advertiser reported that methadone was found in one of the bottles, citing the medical examiner's office.
Irons was on his way from a surfing competition in Puerto Rico to see his pregnant wife in Hawaii when he died.
His family has maintained that Irons, whose first child is due next month, died after contracting dengue fever during a recent trip to Portugal.
"Coming into Puerto Rico, he was fired up and ready to compete," Rarick said, adding that his friend appeared to be in good physical shape and happily signed autographs shortly after he arrived.
But Irons quickly fell ill, Rarick said, and sat out of the first round of competition. He then dropped out of the competition before the second round and was put on intravenous fluids by a doctor for dehydration.
Rarick said the doctor in Puerto Rico had advised Irons to stay on the island for a few more days to get stronger, but that the three-time world champion was eager to get home to his wife because he was no longer competing.
He was feeling better, Rarick said, but became more ill on the plane from Puerto Rico to Dallas. He checked into the hotel room to get a good night's sleep before connecting to Hawaii.
Hotel workers found Irons after he failed to answer a wake-up call.
Irons had returned to the world of competitive surfing this year after taking a year off to deal with personal issues. But Rarick said reports that Irons had battled a serious drug addiction during his time off were "probably overblown."
The Irons' family has said that he told them he'd contracted dengue fever in Portugal. The virus, transmitted by mosquitoes, can cause shock, organ damage and death, if left untreated.
A preliminary autopsy showed no signs of foul play. The medical examiner's office in Texas told ABC News that a toxicology test will be conducted to look for drugs in his system. The results are expected in four to six weeks.
The medical examiner's office would not confirm the report from Honolulu that methadone was found in his room.
Irons admitted he was battling personal issues in the recent documentary, "I Surf Because."
"If I didn't have surfing to get those out of my system," he said, "I would self-destruct."
His death has shaken the surfing community he so loved.
"It is unfortunate and it's really been a shock to the whole world of surfing," Rarick said. "The word spread like wildfire and nobody could believe what happened."