With the packed garden a sea of tricolors, Dublin-born McGregor apologized for taking two rounds to dispatch his overmatched opponent.
He won via TKO at 1:54 of the second round.
The fight had been heavily promoted, including on FOX TV nationwide during the NFC championship football game between Seattle and Green Bay. McGregor was compared to boxing legend Muhammad Ali in the promos.
Referee Herb Dean stopped the contest after McGregor pounded and dropped Siver.
McGregor apologized for not ending the fight in two minutes as he had predicted.
"I said two minutes -- I meant two rounds," McGregor said. "I felt comfortable throughout. I don't think any of these featherweights pose a threat. Every single one keeps talking but I don't hear them talking about skill or technique, because they know I own that."
McGregor also confronted UFC featherweight champion Jose Aldo, who was in attendance. McGregor fight Aldo (25-1) for the 145-pound title later this year.
"It's probably looking like Vegas is the most likely place it's going to happen," UFC CEO Lorenzo Fertitta told ESPN.com. "We kind of went back and forth over what makes sense: Ireland, Brazil, Vegas.
"The reality is the time in Ireland makes it very difficult to draw PPV numbers. We couldn't get the permits that would allow us to do the event early morning, which means we'd have to hold the PPV in the afternoon in the U.S. We didn't feel like that would be feasible. There's nothing wrong with a big fight in Vegas."
Fertitta said the bout would likely take place in May.