The American actor was best known for his role in Seasons 6 and 7 of FX's hit Sons of Anarchy.
TMZ reported his body was found by his girlfriend in the hallway of her Los Angeles apartment on Wednesday evening. He was also apparently struggling with a drinking problem.
He played Hugh, a former IRA member and gun runner, in Kurt Sutter's crime saga. However, there is no official cause of death to report at this time. Following Sons of Anarchy, O'Neill had a minor role in the 2016 film Urge, which starred Pierce Brosnan and Danny Masterson.
Alan made his acting debut in the 1997 TV movie Ballyseedy and went on to have roles in 1989 drama Fair City and the 2001 series Rebel Heart, where he played Keith McGrath.
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The new results represent the longest systematic record of atmospheric methane, with measurements taken regularly over five years. In December 2012, the rover's two-year mission was extended indefinitely. "So way under the ground this methane is trapped".
Added Kristen Renton, who portrayed Ima Tite on the series, "RIP sir..."
News that the actor (legal name Michael Alan O'Neill) was pronounced dead at a residence in Toluca Lake, Calif. on Wednesday, June 6 at 6 p.m.
Irish actress Jenny Dixon, who worked with O'Neill on 2012 short film Invisible, said he was a "great friend who taught me so much". Thank you for your kindness.
Hugh was a recurring character right up until the seventh, and final, season. A great man, sadly missed.