Mr Peter Jackson (41) of Kerryhall Road, Fairhill, Co Cork had pleaded not guilty to murder but guilty to manslaughter for the fatal stabbing of David Hamilton (31), at a house on Kerryhall Road on May 4 2012.
The jury of ten men and two women took nine hours and 54 minutes to come to their decision by a majority of 11 to one, having spent four days listening to evidence.
Ms Justice Isobel Kennedy thanked the jurors and exempted them from further jury service for 15 years.
Mr Jackson will be sentenced on November 23 when victim impact statements will be read to the court.
When the guilty verdict was read at 4.15 this afternoon (Friday), members of Mr Hamilton's family began crying and exchanged hugs. They left the court together soon afterwards.
During the trial, the court heard that Mr Jackson and Mr Hamilton were heroin users who went to a house on Kerryhall Road on May 4 2012 to inject heroin.
Ms Pamela McHale, (26) from Cork, who was a friend of Mr Jackson, told the court that they had bought two bags of heroin earlier that day worth €50 each. An argument started after Mr Jackson had cooked up the first bag of heroin and injected himself and Ms McHale but had difficulty finding a vein to inject Mr Hamilton.
Ms McHale said: "Dave was giving out and Peter was like "calm down" but Dave wanted to go to another fella's house who he said could inject proper."
They began smoking the second bag on a piece of tinfoil but Ms McHale said Mr Hamilton became increasingly agitated and wanted to take the heroin to the other house so that he could inject.
"Peter and Dave got in a fight," she said. "He was trying to pull the foil off Peter and Peter gave him a dig and Dave gave Peter a dig. Peter picked something up from the table and he swung."
At the time she did not see what Mr Jackson had picked up and she said Mr Hamilton left the room and became hysterical. It was some minutes before she noticed that there was "blood everywhere" and that his shirt was soaked with blood. It was around this time that she became aware of the knife that had been used to stab Mr Hamilton.
The court also heard from the owner of the house Mr Pat Fennelly. He said that when he saw Mr Hamilton covered in blood he told Mr Jackson and Ms McHale to get him out. Mr Hamilton left the house and collapsed on the pavement outside while Ms McHale called an ambulance.
Mr Jackson initially told gardai that he had found Mr Hamilton bleeding on the pavement but later admitted his role in the stabbing.
Speaking for the defence Mr Dominic McGinn SC told the jurors that they would have to get "inside the head" of Mr Jackson on the day of the stabbing to assess whether he intended to kill or cause serious injury to Mr Hamilton.
He said that Mr Jackson had been friendly with Mr Hamilton and offered to share the heroin he had bought that day because Mr Hamilton was finding it difficult to get some. He described Mr Hamilton's death as "tragic" and said that Mr Jackson did not act like a killer in the aftermath of the stabbing.
When giving the prosecution case Thomas O'Connell SC told the court that Mr Jackson was not provoked by Mr Hamilton and that the "natural and probable" consequences of stabbing him in the chest were that he would kill or cause serious injury.