Gardaí believe two men were behind the gun attack which left the intended victim and a man whose home he was visiting dead.
Brendan Molyneaux, a man in his mid-40s, who had been living in homeless hostel, was killed alongside Paddy Mooney, 58.
Both men were shot at Mooney’s ground floor flat in Pearse House in Dublin’s south inner city shortly before 7pm on Sunday.
Molyneaux is believed to have been the intended target, while Mooney, who neighbours described as “harmless”, may have been killed after he witnessed the murder.
Yesterday locals described Mooney as a “quiet man”.
One woman said he had lived in the complex for nine years and she was unaware of him being associated with any trouble during that time
Labour Party justice spokesman Pat Rabbitte said: “The three shootings that took place in Dublin over the weekend are a clear indication that the Government has failed to tackle gangland crime, and gun crime in particular.”
Michael Feehan, Assistant Garda Commissioner for the Dublin region, branded the three killings a tragedy and appealed for anyone with information to come forward.
It is believed one of the Pearse House assailants pulled the trigger and killed the two men.
Both assailants are described as being in their 20s, of medium build and average height and were wearing either baseball caps or balaclavas.
Four people are being questioned in relation to the double killing and are being interviewed at Pearse Street, Harcourt Terrace, Kevin Street and Kilmainham Garda Stations under Section 50 of the Criminal Justice Act.
Two of the men are in their mid-40s, while the third man and woman are in their mid-20s.
The murders came a day after the body of 29-year-old John Paul Joyce, a major drug-trafficking figure, was found shot dead in a ditch in the north of the city.
Detectives probing the killing of Mr Joyce believe the murder may be linked to a warring feud between rival drug gangs.
It is believed the settled traveller from Grove Road, on the Malahide Road in the north of the capital, was shot at least twice with a handgun.
It is believed all three men may have been victims of a Finglas-based crime gang. This gang was also responsible for the double murder of its then leading figure Martin ”Marlo” Hyland and innocent plumber Anthony Campbell, 20, in December 2006.
Campbell was killed at that time in order to remove any possible witnesses.