Mixed martial arts star Conor McGregor vowed to knock out boxer Floyd Mayweather on Friday as a horde of Irish fans descended on Las Vegas for the trash-talking rivals' cross-combat super-fight.

McGregor, 29, will carry a substantial weight advantage into Saturday's 12-round boxing contest at the T-Mobile Arena after a raucous weigh-in that saw him face-off with Mayweather for the final time before fight-night.

With waves of green-shirted fans in a crowd of around 6,000 roaring their support for the Irish superstar, McGregor tipped the scales at 153 pounds, well inside the 154-pound limit.

Former welterweight champion Mayweather, 40, who has emerged from a two-year retirement to take on MMA star McGregor, weighed in at 149.5 pounds.

The naturally bigger McGregor — who screamed into Mayweather's face as the two men went nose-to-nose — said he expects to enter the ring at closer to 170 pounds.

Saturday's one-off bout is projected to be the richest fight in history, with Mayweather poised to earn as much as $200 million and McGregor potentially pocketing around $100 million.

McGregor, who has never fought in a professional boxing contest, is a massive underdog for a fight, which is expected to beamed live to around 200 countries and territories across the globe.

But the former apprentice plumber from Dublin, who was unemployed four years ago, defiantly predicted he was ready to spring a monumental upset on the skillful Mayweather, one of the finest boxers in history who boasts a perfect 49-0 record.

“That's the worst shape I've ever seen him,” McGregor said of Mayweather. “I'm going to breeze through him, trust me on that.”

'Fighting wins fights'

A nonplussed Mayweather shrugged off his weight disadvantage and once again vowed to stop McGregor inside the distance.

“Weight doesn't win fights, fighting wins fights,” Mayweather said. “It won't go the distance. Mark my words — I'm not worried about the scales.”

McGregor meanwhile basked in the adulation of his Irish fans who have poured into the Nevada desert gambling capital this week.

“You can't beat us — we've already taken over,” McGregor bellowed. “Las Vegas is Ireland now.” An army of Irish fans poured out onto the Las Vegas strip following the weigh-in, dancing and singing in blazing hot sunshine.

Saturday's bout has appalled boxing traditionalists, who have rubbished the event as a meaningless, money-grabbing, mismatch which owes more to the pantomime traditions of WWE wrestling than the noble art.

Boxing pundits, coaches and fighters have lined up to dismiss McGregor's hopes of succeeding where 49 others in Mayweather's 21-year career have failed.

“Would a ping-pong player have a chance of scoring a point against Roger Federer?” was the withering verdict of veteran trainer Teddy Atlas. “It's going to be like walking through a really bad neighbourhood late at night. McGregor's going to get mugged.”

Safety fears

The hard-nosed heavy-hitters of the gambling world agree.

Saturday's fight is set to smash records as the most bet-upon bout in Las Vegas history.

But while McGregor, a long-odds underdog, has attracted most wagers, the majority of the money is on Mayweather.

William Hill US reported that several large bets had been placed on Mayweather for what is expected to be a straightforward assignment.

One punter placed a $1.2 million cash bet on Mayweather, which will earn around $240,000.

Yet the mere fact that McGregor will carry at least a puncher's chance into the contest has been enough to sustain the excitement.

Stephen Espinoza, the head of cable network Showtime Sports which is selling the fight on pay-per-view in the US, said surveys had shown that fans were not deterred by predictions of a mismatch.

“The casual fans were absolutely adamant,” Espinoza said. “Their response almost universally was 'We don't care if it's a mismatch ... if there's a .01 chance that something incredible could happen, we need to watch it.' And that's why they're going to watch it.”

Others however have sounded a note of caution, citing potential safety risks to McGregor given the chasm of experience between the Irishman and Mayweather.

British former light-welterweight world champion Amir Khan expressed fears Friday McGregor could be seriously injured.

“It's like a tennis player trying to play badminton,” Khan said. “(McGregor) needs to think about himself in this fight, because if he gets seriously injured, he might not ever be the same fighter or he might not even fight again.”