O'Neill's manager, Greg Meyer, confirmed the actor's death on Thursday.
Irish actor Alan O'Neill, who appeared in motorbike gang drama Sons of Anarchy, has died at the age of 47.
O'Neill's girlfriend found him unresponsive in the hallway of their apartment in Los Angeles. He landed a role as Paddy in 2007's "32A", played Robert in 2009's "Moore Street Masala" and starred in a hit United Kingdom crime series called "Inspector George Gently".
Most recently, he played Captain in the 2016 film Urge.
Sons of Anarchy ran between 2008 and 2014 and followed the lives of a fictional outlaw motorcycle club that worked to protect their local community from lawless groups.
Among his other roles, O'Neill played the regular role of Keith McGrath in the drama Fair Cityfrom 2006 to 2012.
A great friend who taught me so much.
She tweeted: "Thank you for your kindness".
"It's with sadness I confirm the passing of Alan O'Neill, a brilliant amusing and kind human being", his agent Annette Walsh writes in a statement. "My thoughts are with his partner, children, family and friends at this time". So very sorry to hear of your passing.
The Irish-born actor first began his acting career in 1997 in the TV Movie Ballyseedy.