The killings are believed to be part of a long-running feud in the area, which has seen a number of shooting and assaults.
It is understood the target of the latest attack is the brother of a man shot dead in Dublin in early 2014.
Gardaí are examining a firearm they recovered. They would not say how many gunmen they believe were involved.
Meanwhile, a second car is being examined in relation to the attack - Golf GTI, registration 06 LH 3466.
Politicians roundly condemned the incident.
Minister for Justice Charlie Flanagan said: "This outrageous attack is totally unacceptable...I utterly deplore this shooting which has tragically resulted in the loss of two lives as well as others injured.
"This callous, cold-blooded violence has no place in our communities - the Government and Gardaí stand with the people of Ballymun against such despicable acts and will continue to ensure there is high visibility policing to combat such attacks, including extensive armed checkpoints across the city, for as long as necessary."