History was made Saturday night at the Garden.

Mixed martial arts made its long-awaited debut in New York City, and a sellout crowd saw a UFC first: a simultaneous two-division champion.

Brash Irishman Conor McGregor, the featherweight champion, moved up to the lightweight division and dominated the champion Eddie Alvarez, stopping him at 3:04 of the second round of the main event at UFC 205.

McGregor (21-3 MMA, 9-1 UFC), too quick and composed for Alvarez, spent the week talking trash, predicting an easy fight and hyping up the historic card. Then he more than backed up his talk against Alvarez, further establishing himself as the sport’s premier attraction.

“You’ve gotta have size, reach, length. They’re not on my level,” he said in his in-ring interview after the victory. “You’ve gotta have attributes.”

McGregor now has won his past two fights after losing to Nick Diaz at the welterweight level in March. He got revenge against Diaz on Aug. 20 in UFC 202 and destroyed Alvarez, who was coming off three consecutive victories, the last a championship victory over Rafael dos Anjos. It was the first time in five UFC fights Alvarez had been knocked out and just the third time in his long mixed martial arts career he failed to get through two rounds.

McGregor, beloved by his fans for his willingness to say anything and insult basically everyone, didn’t hold back after the victory, dropping a few f-bombs.

“I ridiculed everyone on the roster,” McGregor said. “I just want to say from the bottom of me heart, I’d like to take this time to apologize… to absolutely nobody. The double champ does whatever … he wants.”

At the outset of the interview, he wanted to know where his other belt was, accusing the UFC of being cheap.

“This company was sold for $4 billion,” he said.

When asked what was next, McGregor said: “I’m gonna get that second belt. It should have been out here ready.”

UFC president Dana White said it was McGregor’s responsibility to bring his other belt, not his. So they borrowed the welterweight title belt from Tyon Woodley, and later McGregor gave it back to him.

“I’ve never dealt with anybody like this kid on so many different levels,” White said.

Despite the belt snafu, White could say enough good things about McGregor.

“Conor’s special,” he said. “Everybody’s been saying wait until he faces a wrestler, wait until he faces a wrestler. He throws that left hand with no effort, but boy when he touches people, they go. It’s unbelievable.

“He was made for this event.”

UFC has yet to decide if McGregor will be allowed to defend both championships at the same time. He has been the featherweight champion since knocking out Jose Aldo last December.

“My dream has become a reality,” McGregor said after receiving both title belts. “God bless.”

McGregor controlled the first round, twice knocking down Alvarez (28-5 MMA, 3-2 UFC) with forceful left hands. He was accurate and timely with his shots, counter-punching and defending well against Alvarez’s attempts to take him to the ground. Alvarez meanwhile seemed jumpy, missing by wide margins, slipping once.

McGregor started the second by taunting Alvarez — he put his hands behind his back, daring him to be aggressive, a la UFC legend Anderson Silva — then finished him shortly thereafter.

McGregor caught Alvarez with a left hook on the back of the head, followed with a short right hand, sending Alvarez to the canvas. He pounced from there, landing shots before it was appropriately stopped.

McGregor entered the octagon to 50 Cent’s “I Get Money.” The key line? “I run New York.”

With the support of the city’s Irish community, he carried the event all week, hyping up the card like only he can. The final gate for the night was $17.7 million, smashing both the UFC and Garden records, and the attendance was 20,427.

McGregor, who confirmed that he and his partner, Dee Devlin, are expecting a child early next year, said he wants a piece of the UFC.

“Where’s my share, where’s my equity,” he said. “They’ve got to come talk to me now. … I’m aware of my worth. I’m coming for men now if they want me back.

“I feel like I’m only reaching my prime.”

He arrived late for Thursday’s press conference in a full-length Gucci white mink coast and waved it around the room. The two put on a show, Alvarez tossing a chair toward McGregor, who had to be carried off the stage or he would have retaliated.

It was merely a preview for the big show Saturday night. That didn’t disappoint either.