Two men remain under Garda custody in connection with the murder of Eamon Kelly.
The two men are being detained in Coolock and Raheny Garda stations.
Two women and two men were released without charge in relation to the murder during the weekend.
Sean Connolly, of Bernard Curtis House in Bluebell, south Dublin has been remanded in custody accused of murdering the 65-year-old convicted drug dealer.
Kelly was a career criminal dubbed 'the Godfather'. He was shot dead in broad daylight near his home in Furry Park Road in Killester, Dublin. He escaped a previous attempt on his life in 2010 when his attacker's gun jammed.
His 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 (2.2lb) 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 gang leader Eamonn 'The Don' Dunne, who was shot dead in a pub in Cabra, Dublin, in 2010.