AceShowbiz - Sons of Anarchy actor Alan O'Neill has died at the age of 47.

The Irish-born star, who appeared as Hugh in seasons six and seven of the hit series, was found unresponsive by his girlfriend at his Los Angeles apartment on Wednesday evening, June 06, according to TMZ.com.

O'Neill reportedly had a history of heart problems and battled drug and alcohol addiction, but a cause of death has yet to be determined.

"It's with sadness I confirm the passing of Alan O'Neill, a brilliant funny 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... May he rest in peace."

O'Neill began his career onstage, starring in productions of King Lear and Hamlet. He later moved on to small screen roles like Inspector George Gently in the Irish series Fair City from 1989 to 1994.

He recently filmed a role in the yet-to-be-released Manson Family horror movie Charlie Lives: The Family's Return.