A man in his twenties was arrested in Artane in Dublin this afternoon by gardaí investigating the murder of Alan Ryan.
He is being held under Section 50 of the Criminal Justice Act, 2007, at Coolock Garda Station.
This is the fourth arrest as part of the investigation into the killing.
Ryan, who had served time in prison for firearms offences linked to his membership of the Real IRA, was shot on the afternoon of September 3rd at Grange Lodge Avenue, Clongriffin, north Dublin.
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In recent years he had emerged as a key player in the organisation in Dublin, fundraising by extorting publicans and drug gangs.
Gardaí believe one of the Dublin organised crime gangs with whom he had come into conflict was behind his murder.