The suspect, a man in his 20s from Finglas, is also suspected of carrying out another gangland murder last year.
He was arrested at his home with his girlfriend, in the hours after the shooting of criminal Brendan Molyneaux and innocent man Paddy Mooney at Pearse House last Sunday.
Armed gardai stormed the property and detained the pair at gunpoint.
The man is an associate of the Finglas criminal nicknamed The Don.
Gardai suspect that his gang was behind the double killing, and the murder of another man, John Paul Joyce, whose body was found 24 hours earlier.
Two close associates of the chief suspect were picked up walking through the city centre at the same time as the Finglas house was raided.
They are from the south inner city but are linked to the Finglas gunman.
Gardai believe the suspect carried out the shooting of Michael Murray (41) in Finglas in March 2009. Murray was involved in a feud with the Don's gang and was targeted because gang members believed he was preparing to target them.
The suspect, who is currently on bail for other crimes, is described as "highly volatile".
He is expected to be held for questioning for several days. Under the law all four prisoners can be quizzed until Sunday evening.
They all remained in custody today.
Detectives believe the man carried out the killing at the behest of The Don, after rumours surfaced late last year that Brendan Molyneaux had agreed to target the gang boss, or another senior member of the Finglas gang, to raise money.
Michael Murray was murdered in similar circumstances nine months ago.
Murdered Molyneaux, who was a heavy drinker and had lived in a hostel since his release from prison, has been described as "an unlikely hitman" by sources, who add that he was not experienced enough with a handgun to carry out any such attack.
The gang behind the killing, which is led by The Don, are suspected of being behind 11 murders over the past four years, including those of gang boss Marlo Hyland and innocent plumber Anthony Campbell.
Gangland tensions are running high in north Dublin following the murder of the two men in the city centre, and the killing of criminal John Paul Joyce, whose body was discovered shot and dumped at Dublin Airport last Saturday.
It is believed several gang members are living in fear for their lives following the murders of drug dealer Joyce, as well as the double murder of Brendan Molyneaux and his friend, innocent victim Paddy Mooney.
Joyce was shot dead after being abducted last week. His body was found at Dunbro Lane, near Dublin Airport, last Saturday.
It is understood that detectives have informed several men that a series of threats have been made to their lives -- at least three gangland figures in north Dublin are now wearing bullet-proof vests.
One of them, who is a well-known member of a northside gang and a one-time pal of murdered John Paul Joyce, was stopped by gardai between Christmas and New Year, and told officers he was running scared as several attempts had been made on his life.
The man's brother has also been shot at, in what was a case of mistaken identity. This man, who is also a former associate of slain drug dealers John 'BJ' Clarke and David 'Fred' Lynch, wears a bulletproof vest, and flits from house to safe house to try to avoid detection.
Gardai are continuing to search for the handgun used in the killing, and are examining CCTV from the Pearse House area, Lower Sandwith Street and Hanover Street East.