Conor McGregor has returned to action with a stunning knockout of Donald "Cowboy" Cerrone in under a minute at UFC 246, in his first mixed martial arts fight since losing a lightweight title bout to Khabib Nurmagomedov 15 months ago.

The former two-division champion left Cerrone bloodied just seconds into Saturday's welterweight fight by ramming his shoulders into his face from the clinch. When they broke, McGregor landed a head kick that set the American wobbling and scrambling back to the cage.

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The 31-year-old Irishman closed in and rained down strikes as Cerrone, 36, covered up, prompting referee Herb Dean to stop the fight after just 40 seconds, in a night that echoed McGregor's greatest fights during his unparalleled rise.

"I feel really good, and I came out of here unscathed," McGregor said in his post-fight interview. "I'm in shape. We've got work to do to get back to where I was."

McGregor lands a knee to the face of Cerrone in the first round of the welterweight bout during UFC246 [Steve Marcus/Getty Images/AFP]

McGregor served a six-month suspension and was fined $50,000 for his role in a brawl at the end of his October 2018 loss to Nurmagomedov.

He was convicted of assault late last year for punching a man who had refused a shot of whiskey from him in a Dublin pub while, in a separate case in 2018, he was charged with three counts of assault and one count of criminal mischief after police said he attacked a vehicle carrying UFC fighters. He pleaded guilty to a reduced disorderly conduct charge in that case.

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The victory was McGregor's first since November 2016 at Madison Square Garden where he beat Eddie Alvarez to win the lightweight title and become the organisation's first simultaneous two-weight champion.

With his fame and fortune multiplying, McGregor fought a boxing match with Floyd Mayweather in 2017 before losing to Nurmagomedov.

"I wasn't committed," McGregor told reporters after his win. "I just felt like I disrespected the people that believed in me and supported me. That's what led me to re-centre myself and get back to where I was at," he added.

McGregor has said he planned to have three UFC fights this year.

A crowd of more than 19,000 packed out the T-Mobile Arena in Las Vegas on Saturday and an uninspiring undercard was quickly forgotten as McGregor made his way to the Octagon.

Fans waving Irish Tricolours roared support for their man as the introductions took place and McGregor leaped into action as the fight began.

Cerrone had made his name by going toe-to-toe with some of the heaviest hitters in the UFC, giving fans the fights they want to see, but he was caught cold by McGregor's unorthodox shoulder strikes.

"I had never seen anything like that," Cerrone said. "It threw me way off guard. He busted my nose and it started bleeding and then he stepped back and head-kicked me."

Despite a third straight defeat, Cerrone said he had no plans to retire.

"I'm going to keep fighting man. I love this sport."