Gerard Hutch, known as 'The Monk', has been arrested in Spain on foot of a European Arrest Warrant.
The 58-year-old is currently in Spanish custody.
It is believed he was detained in Fuengirola on the Costa del Sol.
Gardaí believe he had been moving between Spain, where he has a home, and other European countries.
His extradition was sought by the Garda team in Ballymun, which has been investigating the murder of David Byrne at the Regency Hotel in Dublin in February 2016.
The 34-year-old Kinahan organised crime group member was shot dead at a boxing weigh-in by a gang of armed men dressed as gardaí.
A file on the case was submitted to the Director of Public Prosecutions, who then applied to the High Court for a European Arrest Warrant for Gerard Hutch.
Gardaí believe the Regency attack was carried out by the Hutch organised crime group in retaliation for the murder of Gary Hutch, the nephew of Gerard Hutch, in Spain in September of 2015.
They also believe the target was Daniel Kinahan, the controller and manager of the Kinahan organised crime group who was at a boxing weigh-in.
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Kinahan and other senior gang members survived the attack, but it resulted in a dramatic escalation of the Hutch-Kinahan feud, which has so far cost 18 lives
Gerard Hutch left Ireland soon after the Regency attack and has only been seen here once since - at the funeral of his brother Eddie, another victim of the feud.
Gerard Hutch can contest his extradition or waive that right and return voluntarily to Ireland to face trial here in connection with the attack at the Regency.