O'Neill's manager, Greg Meyer, confirmed the death of the actor, who was born in Ireland.
"He was a great guy who was very funny and engaging as well as a very talented actor," Meyer says.
Timothy V. Murphy, who played Galaan, another Irish weapons trafficker and former True IRA member on the FX biker drama, expressed condolences in a tweet.
"My good friend Alan O'Neill died today....we worked together on The Sons of Anarchy...my condolences to his family," he tweeted.
TMZ, quoting unnamed law enforcement sources, reports that O'Neill's girlfriend found his body in the hallway of her Los Angeles apartment on Wednesday night. According to TMZ, the actor was a heavy smoker with a history of heart problems, and foul play is not suspected.