The other murder victim is a man in his 30s.
Another man and woman were treated for minor injuries at the scene.
The shooting took place at a house at Balbutcher Drive at approximately 4pm.
It is understood the intended target, an associate of Mickey Devoy, fled the scene on foot.
Two gunmen are reported to have approached the house with a handgun and a submachine gun.
Gardaí were last night examining a silver Opel Zafira, registration 08 LS 3101, which was found in Balbutcher Drive, as well as a grey Volkswagen Golf GTI, registration 06 LH 3466, which was found in Santry Close. A gun was later recovered in the area which gardaí were also forensically examining.
The shooting is not thought to be related to the Kinahan-Hutch feud but is instead linked to an unpaid drug debt of €30,000 which has caused a dispute between associates of Mickey Devoy and a south inner city gang.
Mickey had approximately 70 convictions before he was shot dead in 2014. He was also the main suspect of a 2005 murder.
His mother Anne ‘Nancy’ Devoy appealed to her son’s murderers at the time, saying she was not angry at them but “hurt” and how they had got the wrong man.
“I know who killed my son. They shot the wrong man. I’m not angry, I’m just hurt.
“I have nothing against the men that done this whatsoever.
“Just please say you are sorry, that they have done the wrong chap,” she said at the time.
Local Sinn Féin councillor Noeleen Reilly told the Irish Examiner that Balbutcher Drive was a busy residential area with young families as well as elderly people, but they would not be “used to” anything like this. She called for more gardaí in the area.
Anyone who was in the area at the time is asked to contact Ballymun Garda Station on 01 666 4400.