Donnelly shot father-of-one Edward Flanagan in the back of the neck as the victim watched a film in his apartment while drinking a can of beer. The incident occurred on June 16, 2011.
Before sentencing, Mr Justice Paul Carney asked Detective Garda Shay Woods if there was a motive, as none was given during the trial.
Det Garda Woods said it appeared Donnelly just wanted the victim and two other men to leave his apartment and they refused.
Speaking after sentencing, Ms Flanagan described her brother's death as a brutal murder and said she did not accept Donnelly had no recollection of events.
She described her brother and his killer as "drinking buddies", insisting they were not best friends as the trial had previously heard. She added how her brother would never get into arguments. She said it was cruel that his 12-year-old son would have to deal with the loss of his father and as a family they were finding it very hard to come to terms with.
"He was the most easygoing, laidback person you'd ever meet," she said.
Donnelly had also admitted possession of a Baikal 9mm automatic pistol and three rounds of .38 calibre ammunition at the same address on June 15, 2011, although he had denied these charges at the start of the trial.
The jury returned with a 11/1 majority verdict of guilty of murder after five hours and 20 minutes of deliberation.
Mr Justice Paul Carney sentenced Donnelly to life imprisonment for the murder and five years on each of the other counts to run concurrently and he backdated it to June 16, 2011, for time spent in custody.
The court heard that Donnelly and Mr Flanagan were drinking cans in a field behind Belcamp College in Coolock with two other men on the day of the incident.
They later went to Donnelly's apartment in nearby Santry to continue drinking and watched two films -- Snatch and Lock, Stock And Two Smoking Barrels.
After the films had finished playing they listened to the radio and Mr Flanagan and Donnelly started dancing.
Gary McMahon, who was one of the other men in the apartment, said an argument started between Donnelly and Mr Flanagan.
He said Donnelly was running around slagging Mr Flanagan and he recalled him saying he would shoot him.
Mr McMahon said that Donnelly walked around the back of the couch, put a gun to Mr Flanagan's head and was shouting in a rage in an English accent just before he shot him.
Gardai had to force open the door of the apartment and emergency services tried to revive Mr Flanagan, but he was later pronounced dead.