McGregor, UFC's hottest property, took the decision to make the 25lbs step-up from featherweight to welterweight Saturday when he came up against Nate Diaz.
Yet by the end of the night in Las Vegas, featherweight champion McGregor, known as the "Notorious One," was on the losing end of a fight for the first time in his UFC career as Diaz forced a tap-out in the second round to claim victory.
"I stormed in and put it all on the line. I took a shot and missed," the Irishman posted on Instagram after the UFC 196 fight. "I will never apologize for taking a shot.
"I'll take this loss like a man. I will not shy away from it. I will not change who I am. If another champion goes up two weights let me know."
"I've been here many times in my life in some form or another," he added. "I'll eat it all and come back stronger."
McGregor's fight with Diaz came about only after Rafael dos Anjos pulled out of the UFC 196 event due to injury 12 days before he was due to defend his lightweight title against the Irishman.
The new contest with Diaz was then moved from 155lbs to the 170lbs limit.
McGregor is expected by some to return to the featherweight division, although he hopes to come up against Diaz again in the future.
"Never ever shy away from challenges," he wrote. "Never run from adversity. Face yourself head on.
"Nate I will see you again."
Diaz's reward for victory could bring him a rematch against Dos Anjos with the lightweight title on the line, having lost to the Brazilian previously in 2014.
The American had less than two weeks to get ready for the fight with McGregor and feels had he been able to prepare fully then his victory may have been a more comprehensive one.
"I knew I was the superior boxer and the superior martial artist," Diaz told UFC's official website
"Nothing surprised me except that I got hit at all. I think with a full camp I would have been flawless."
"There's a new king right here," Diaz added.
Despite suffering defeat against Diaz, McGregor is reportedly set to pick up a $1 million fight purse -- the biggest in UFC history -- as well as a cut of the fight's pay-per-view revenue.
"If you're tired of me talking money, take a nap," McGregor added. "I'll still be here when you wake up with the highest PPV and the gate."
McGregor's defeat was not the only shock in Las Vegas as Holly Holm lost her bantamweight title to Miesha Tate.
Holm had won the belt after defeating Ronda Rousey in November but was unable to repeat those heroics this time around, being choked unconscious by fellow American Tate in the fifth round to suffer her first ever UFC loss.