“FIRST CAR I EVER OWNED?” Conor McGregor asks. It’s a wet autumn afternoon in Dublin, Ireland, and the 27-year-old mixed-martial-arts fighter is sitting on a couch at the Straight Blast Gym, a cavernous training center in his hometown.

“An Opel Astra,” McGregor says (think: Chevy Cruze). “I was an apprentice plumber, and I went into a car place with, like, ten cents in my pocket. I was on nothin’ a week, workin’ my bollocks off. But I went in and they said, ‘We’ll give you this brand-spanking new car if you sign this sheet. And all you gotta do is pay this amount every month.’ And I was like, ‘Sure! No problem!’ Signed the sheet, drove out in the car…never paid a once. They came and got it and took it back and that was that!”

Outside the gym is McGregor’s newest car, a midnight-black Mercedes S500 AMG Coupe, which he has parked diagonally, across two spaces, the kind of aggro-alpha maneuver you can only get away with if your name and face are on the building, which McGregor’s are. The S500 AMG starts above $150,000, and it’s safe to assume McGregor’s is not the starter model. When I asked about it, McGregor reached into his Louis Vuitton duffel and held out the keys like he was offering me potato chips.

“You want to drive it?” he asked.

I get the feeling I could drive off with it and McGregor might not notice for a few weeks. In addition to the Mercedes, he says, he has a Cadillac Escalade, a Range Rover, a BMW 4 series and a BMW 5 series, and a couple weeks after our meeting, he texts me a photograph of his new BMW i8, alpine white, its scissored doors raised like pterodactyl wings.

“Buildin’ a proper fleet,” he says.

The fleet is growing fast, like everything else in the Conor McGregor business. Three years ago, the Dublin native was an unknown tangling with unknowns in the backwaters of mixed martial arts, an exploding if divisive sport that combines multiple disciplines (boxing, wresting, jiu-jitsu, among others) in a (nearly) no-holds-barred cage fight (I’ll stop pretending that you don’t know what I’m talking about). Today, the ex-plumber is one of the biggest names—and most lucrative draws—in the Ultimate Fighting Championship, or UFC, the MMA’s gladiatorial major league. McGregor is the current interim featherweight champion, and on Dec. 12 he intends to strip the “interim” with a win against Brazilian legend and current champion Jose Aldo.

Conor McGregor will compete against Brazilian legend and current champion Jose Aldo on Dec. 12. Photo: Eric Ray Davidson for The Wall Street Journal

“One round,” McGregor bluntly predicts. “Four minutes. I’ll KO him.”

Welcome to the Conor McGregor experience. As he has ascended, McGregor has emerged as one of the most amusing/contentious mouths in sports, a relentless chatterbox who nicknamed himself “the Notorious,” and never met a taunt or boast he didn’t like (he’s referred to other MMA fighters as “schmucks,” and told Esquire magazine he could “kill” Floyd Mayweather in “less than 30 seconds”). McGregor’s mastery of the 21st century lingua franca of confrontation (the jet fuel of social media) has surely contributed to his rise. He talks openly of wanting the “nine-figure” income typically reserved for Super Bowl quarterbacks. His noisy ambition has built an undercurrent of resentment among other UFC fighters. McGregor doesn’t care.

“Let them believe it’s talk,” McGregor says. “There’s a reason why I talk this way. It’s because I’m supremely confident in my abilities.”

Loudmouthing is a cherished part of the fight trade—it’s practically part of the job description, since fighters are expected to promote their matches, especially if they hope to translate interest into lucrative pay-per-view buys. Muhammad Ali raised braggadocio and head games to an art form, but when done poorly, talk can turn an audience off. Charisma is essential. You can’t come off as a mere blowhard when you’re denigrating your opponents—there has to be a twinkle of self-awareness that the audience can pick up on.

McGregor has mastered this subtle wink. I don’t want to say he’s effortlessly charming because he’s not always effortlessly charming—his barbs can be crude and his language is sometimes unprintable—but there’s an irresistible quality to much of his ranting. Maybe it’s the smooth Dublin brogue. Maybe it’s his finely tailored clothes, which provide a jarring contrast to the fearsome, heart-eating gorilla tattooed on his chest. When McGregor appeared in a three-piece suit on Conan O’Brien’s show this summer, he came off less like a cage-fighting brute and more like fair-haired, Rat Pack-y Dean Martin. “Really, I cannot hold any grudge towards him,” McGregor said of Aldo, who had recently pulled out of their July match, citing an injury. “I would not want to face me, either.”

McGregor has emerged as one of the most amusing/contentious mouths in sports. ‘He’s a fascinating character,’ says UFC President Dana White. ‘He’s like nothing we’ve ever seen.’ Photo: Eric Ray Davidson for The Wall Street Journal

John Kavanagh, McGregor’s trainer and the owner of Straight Blast, says McGregor has displayed his trademark magnetism since the day he showed up at the gym as a lean 16-year-old. What McGregor increasingly appears to be—and will be should he defeat Aldo on Dec. 12—is something the UFC covets: a mainstream superstar. For all its success and major sponsors, mixed martial arts is still viewed by some as a sport on the fringes—it’s currently banned in New York state, ruling out a match at Madison Square Garden. Ronda Rousey is becoming a revolutionary UFC star thanks to devastating knockouts, movie roles and a bestselling autobiography, but McGregor—who recently sparred with the 400-pound strongman Hafþór Júlíus Björnsson, aka the Mountain from “Game of Thrones”—is clearly the league’s next big bet.

“He’s a fascinating character,” says UFC president Dana White, who cast McGregor as a coach in the league’s reality TV vehicle, “The Ultimate Fighter.” “He’s like nothing we’ve ever seen. Not only in calling the rounds in which he will win, but the mental warfare. He’s the best at mental warfare and pushing buttons and getting in your head and winning a big piece of the fight before the fight even happens.”

That warfare was on wide display at the UFC’s September “Go Big” press conference, where the promotion assembled its top names in Las Vegas, stacking them in tiers. McGregor, seated in front in a pair of amber aviator sunglasses, commandeered a microphone and stole the show, taunting opponents past and future and proclaiming himself the sport’s financial rainmaker.

“I can make you rich,” McGregor barked at the UFC lightweight champion Rafael dos Anjos. “I change your bum life. You fight me, it’s a celebration. You ring back home. Your ring your wife: ‘Baby, we’ve done it. We’re rich, baby. Conor McGregor made us rich. Break out the red panties. We’re rich, baby!’ ”

Over the top? Definitely. Obnoxious? For sure. But Ariel Helwani, a veteran MMA journalist who covers the UFC for Fox Sports 1, among others, saw ingeniousness in McGregor’s outburst. McGregor’s verbal tangles goosed fan interest and created a list of antagonists to battle. “He did exactly what he meant to do in that press conference—to set up those future fights,” Helwani says.

“It was a stroke of brilliance.”

“I simply spoke fact,” McGregor tells me. “If you went from a $48,000 paycheck to a half-million paycheck, would you not celebrate? Would you not ring your wife? I know I f—ing would. I know I did.” (The UFC declined to confirm these figures).

Conor McGregor Photo: Eric Ray Davidson for The Wall Street Journal

McGregor has a longtime girlfriend, Dee Devlin, who’s been with him since his apprentice-plumber days, and hopes to have children (“I feel I’ll spawn a few world champions”). He has no immediate plans to follow Rousey to Hollywood (“Acting is a weird game”). He also has no intention of growing old in the UFC. “Really, I could walk away now,” he says. “I certainly won’t be a 40-year-old man.”

It’s a cliché of clichés to say the fight business is tough. But what sometimes gets lost in the predictable uproar over the UFC’s bloody combat is the extraordinary demands upon its athletes, who are often mischaracterized as thudding barbarians. Strength, speed, endurance, intelligence—it’s all tested in the octagon, perhaps more than in any other sports environment, and weaknesses are mercilessly exposed.

The drive to be great—Kavanagh says his fighter is “driven to be the best of all time”—consumes McGregor’s life for now. “I’m so lost in it that I can’t imagine anything else,” he says softly. “Like Vincent Van Gogh. He lost his mind in his game. I have lost my mind also. But I am happy with that. I feel to reach the high pinnacle in anything you do, you must almost become insane to what you are doing.”

Whatever mind space he’s in, it is stirring, connecting, succeeding—diabolically, brilliantly. On his way out of the gym, McGregor poses for photographs and then heads for that beautiful Benz lounging across two parking spaces. For a moment, Conor McGregor is quiet and alone. Then he hits the pedal and roars away.

Write to Jason Gay at Jason.Gay@wsj.com