In a release Tuesday, NBA president of league operations Byron Spruell said Irving was being fined for "making obscene gestures on the playing court and directing profane language toward the spectator stands."
Following the Nets' 115-114 loss, Irving said he was responding to words he heard from the crowd and that he would have "the same energy" for the fans that they had for him.
"When people start yelling 'p---y' or 'b----' and 'f--- you' and all this stuff, there's only but so much you take as a competitor," Irving said. "We're the ones expected to be docile and be humble, take a humble approach, f--- that, it's the playoffs. This is what it is."
Game 2 of the series is Wednesday night.
Irving, who played for Boston for two seasons from 2017 to 2019, said Sunday that he has gotten used to getting booed by Celtics fans since leaving the team. Last season, Irving had a water bottle thrown at him by a person in the crowd as he made his way off the floor.
He also said last season that there was "subtle racism" from the TD Garden crowd at times, acknowledging that he heard some racist comments.
Kevin Durant, who has had his own high-profile team changes during his NBA career, shared his perspective Tuesday on how fans react to players they used to root for, saying "It's healthy once everybody understands both sides."
"It's rooted in love," Durant said. "They once loved you. They once cheered for you and bought your merchandise and had life-altering experiences coming to games watching you play. So when that kind of gets ripped from them from just something like a trade or demanding a trade or wanting to lead, they feel like a piece of them is gone too."
Asked if he thought Irving enjoyed playing the villain role, Durant said, "Some days he might be up for it; some days he might not. But he understands what this job entails. We understand what this situation is. So he might not be in the mood for it next game, who knows."
Nets coach Steve Nash was asked after Wednesday's shootaround whether he would like to see Irving not respond to Boston fans moving forward. Nash paused for a couple moments before saying, "I don't care."
"What did he have?" Nash said.
"39," reporters responded.
Nash shrugged and smiled a little before saying, "I'll keep it at that."
Nets swingman Bruce Brown, who is a Boston native, said Tuesday that he doesn't think Irving is concerned about the reception that is likely awaiting him from Celtics fans in Game 2.
"The Celtics fans are loud," Brown said. "They can be a little much sometimes, but it's Boston. A great fan base. They'll be on him [Wednesday], but he ain't too worried about it."
ESPN's Nick Friedell contributed to this report.