Alan O'Neill has passed away aged 47, his agent has confirmed.
Law enforcement sources said the 'Sons of Anarchy' star's body was discovered by his girlfriend in the hallway of their Los Angeles apartment on Wednesday night.
According to TMZ, Alan - who played former True IRA member Hugh on the FX series - had a history of heart problems and was a heavy smoker, and he is also said to have a history of drug and alcohol abuse.
An autopsy is expected to be carried out soon to determine how the actor died, but no foul play is suspected.
Alan's agent Annette Walsh led tributes to the "brilliant, funny and kind" late star.
She said in a statement: "It's with sadness I confirm the passing of Alan O'Neill, a brilliant funny and kind human being.
"My thoughts are with his partner, children, family and friends at this time. I'll personally miss knowing Alan isn't sharing the planet with us any longer, despite the miles between LA and Dublin.
"Alan was only ever a phone call away. May he rest in peace."
Several stars took to social media to pay tribute to Alan.
'Road to Paloma' actor Timothy V. Murphy wrote: "My good friend Alan O'Neill died today....we worked together on The Sons Of Anarchy...my condolences to his family @SonsofAnarchy (sic)"
Seamus Power tweeted: "So sad to hear of the passing of Alan O'Neill who played "Keith" in Fair City. Always enjoyed working with him. Rest In Peace old pal. (sic)"
Perez Hilton wrote on Twitter: "So young. Alan O'Neill battled many demons in his life. And they got the best of him. (sic)"
Alan starred as Keith McGrath in Irish drama' Fair City' from 2006 to 2012.