The 'Tudors' star - who has been in rehab five times for alcohol problems - met the people at a pub in Brixton, South London and continued partying with them at a private residence where he reportedly downed cans of cider and attempted to play Bob Marley's 'No Woman No Cry' on a guitar.
A source at the party told The Sun: "He was drinking pints of cider and was off his face."
Last month the troubled Irish actor refused treatment by paramedics for more than 30 minutes, meaning they were forced to call police and he was eventually taken away by ambulance to a facility near his home in Maida Vale, North London.
A witness said: "This is very sad. It was the opinion of those present that he tried to take his life."
Worried neighbours saw the 33-year-old actor - who was believed to have overdosed on unspecified pills - rushed to hospital.
One onlooker explained: "I saw the ambulance arriving and was worried to death.
"Jonathan is a lovely guy. You see him all the time in the street. He says hello and is very pleasant."
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