Stephen Boyle had admitted killing but denied murdering Paul Boyle (19) at Early’s Bar, Leabgarrow, Arranmore on October, 2009.
Boyle, 41, Cambridge Road, Kilburn Park in London had pleaded not guilty to murder but guilty of manslaughter.
The jury of six men and six women returned a unanimous verdict of not guilty of murder but guilty of manslaughter in just over two-and-a-half hours of deliberation.
The prosecution asked for time for preparation of a victim impact report before sentencing on March 12 next.
Judge Patrick McCarthy had told the jury one of the issues they had to decide was whether the accused was not guilty of murder but guilty of manslaughter by reason of diminished responsibility.
He told them they also had to consider the issue of self-defence in which the prosecution would have to prove the accused did not use a reasonable amount of force and said there was a grey area there for juries.
“The prosecution must prove the killing was unlawful but by pleading guilty to manslaughter the accused accepts this is an unlawful killing”, he added.
He told the jury the prosecution was required to prove more than the physical action in question and he said they must prove that he had a certain state of mind at the time, a guilty mind.
The incident happened around 2.30am and Paul Boyle was brought to the mainland by lifeboat but he died in Letterkenny Hospital at 10.20am on October 3.
Mr Paul O Higgins SC prosecuting said the killing had caused immense shock and sadness on the island.
Dr Khalid Jabbar told the jury Paul Boyle died from associated blood loss and trauma caused by sharp force trauma.
He said he carried out the post mortem on October 4, 2009 after he was given a history the man had been assaulted with a glass.
Dr Jabbar told the court there was trauma to the left side of the neck, which was caused by sharp force trauma and said he lost 60% of his blood.
Consultant psychiatrist Dr Mary McGuire told Mr Michael O Higgins SC defending that the accused suffered from bipolar II disorder at the time of the incident, which diminished his responsibility.
She said the accused was referred to her on October 21, 2009 at Castlerea Prison where she met with him around 25 times.
Dr McGuire told the court that on the night of the incident the accused said he swung around and hit Paul Boyle with a pint glass and has expressed remorse about it.
Witness Brendan Gillespie told Mr Paul O Higgins that Paul Boyle was stuck twice by the accused with a glass, which he said broke after the second blow.
Under cross-examination, he told Mr Micheal O Higgins when Paul Boyle was struck first, his head went forward but said: “I’m sure he didn’t mean to cut him in the neck.”