The 25-year-old was shot as he sat in a car at McKee Road, Finglas, at about 15:15 local time. RTE has reported he was a well-known dissident republican.
The victim was approached by another car and a number of shots were fired. It is believed he was hit in the head, throat and chest.
The car used by the attackers was then driven from the scene.
A burnt-out vehicle was found nearby.
RTE has reported that the man was well-known to police for his involvement in dissident republican activities.
It said he had been warned that his life was in danger.
A close relative - also involved in dissident republican activity - was shot dead in 2012.
Police are not linking Monday's murder to a gangland feud in the city that claimed two lives earlier this month.
A man critically injured in a gun attack in north Dublin on Monday afternoon has died in hospital.