Dubliner McGregor was in typically brash form as he predicted he would shock the sporting world and knock Mayweather out inside four rounds as he makes a highly ambitious boxing debut, with even the suit he wore to the Las Vegas press conference taunting his unbeaten rival.
Making reference to Mayweather's battles with the American taxman - he is said to owe more than $22m (£17.1m) - McGregor silenced the Michigan-born brawler.
The relationship between Helwani and UFC President Dana White has been well-documented, with White trying to limit Helwani's ability to cover mixed martial arts for MMAFighting. "The taxpayer has a significant liquidity event scheduled in about 60 days from which he intends to pay the balance of the 2015 tax liability due and outstanding".
"He is f*****. His little legs, his little core, his little head". I'm going to knock him out inside four rounds, mark my words. The power and ferociousness that I have, he's never experienced it before. You misadvised him. You should have kept him retired.
"I don't fear him, I don't fear his limited set of fighting".
"I'm a young, confident, happy man. I've worked extremely hard for this. If you zoom in on the pin-stripe it says f*** you!"
The publicity tour moves on to Toronto on Wednesday, New York on Thursday and finally London's SSE Arena on Friday.
"He looks good for a seven-figure fighter, he looks good for an eight-figure fighter, but I'm a nine-figure fighter".
Floyd Mayweather: You're going out on your face or you're going out on your back. Which was do you want to go?