The undefeated boxer and the two-division UFC champion weighed in comfortably below the 69.9kg limit before a raucous crowd at T-Mobile Arena, which will host the so-called Money Fight on Saturday night (Sunday afternoon AEST).

The enthusiastic crowd included thousands of McGregor's vocal Irish fans, who sang and waved flags.

Mayweather was the villain as usual, but his cheering section also appeared to be more robust than normal for the American antihero.

Mayweather had expressed doubt McGregor could make the weight, but the Irishman roared and flexed as he came in well within the mark.

McGregor is making his professional boxing debut in Las Vegas.

"I'll stomp my foot in the centre of that ring, and I won't go nowhere," said McGregor, who expects to weigh around 77.1kg by the first bell.

Mayweather (49-0) came in remarkably light, and he is likely to have a noticeable size disadvantage.

Although Mayweather had a bulging six-pack, McGregor claimed he wasn't impressed.

"He looks blown out and out of shape," McGregor said. "That's the worst shape I've ever seen him in."

Mayweather expressed no concern about his own weight disadvantage.

The 40-year-old champion's game is built on speed and elusiveness, not brawn.

"Weight doesn't win fights," he said. "Fighting wins fights."

Mayweather also wasn't bothered by the crowd booing him.

"I've been here before," Mayweather said.

"I know what it takes in a fight of this magnitude. He did a lot of [talking]. I did a lot of this. Tomorrow, it comes down to the fighters."

Ticket prices for Saturday's show are trending down, with many below the original list price.

Some tickets at the T-Mobile arena could be had for as little as $1,390, while seats closer to the action were being listed on secondary markets for less than they originally cost.

A day before the fight, there were also hundreds of tickets left at the box office.

While ticket sales have been spotty, the fight is still expected to do huge numbers on pay-per-view.

Up to 50 million people are expected to watch the bout in the United States alone.

AP