Three men and a woman remain in custody in connection with the murder of Eamon Kelly.
The 65-year-old convicted drug dealer was gunned down close to his home in north Dublin on Tuesday. He was one of Ireland's most notorious criminals.
Gardai arrested six people on Saturday for questioning. A spokesman said a man and woman who were being held at Clontarf Garda Station have been released without charge.
"A file will be prepared for the Director of Public Prosecutions," he said. "Four people remain in custody, three males and one female." They can be held for another two days.
Kelly, a career criminal dubbed "the Godfather", was shot dead in broad daylight near his home in Furry Park Road in Killester.
Suspected dissident republican Sean Connolly, 34, of Bernard Curtis House in Bluebell, south Dublin, was charged with Kelly's murder on Friday night. He was also charged with possession of a Bloc semi-automatic pistol with intent to endanger life and Real IRA membership on Furry Park Road on December 4.
He will reappear before the three-judge non-jury Special Criminal Court via video link on December 20.
Kelly was the 15th gun murder victim so far this year. At least one other man is suspected of being involved with his killing.
Kelly was jailed for 14 years in the 1990s for possession of 1kg of cocaine and was sentenced for assaulting a Workers Party member in the late 1980s.
In recent years he has been portrayed as a close associate and confidant of ruthless gang leader and drugs boss Eamonn "The Don" Dunne, who was shot dead in a pub in Cabra, north Dublin, in 2010.