Gerard Hutch, the man known as the Monk, has lost his appeal against extradition to Ireland at a national court in Spain.
The ruling by the three judges was published this afternoon.
Gerard Hutch was arrested in Fuengirola on the Costa Del Sol in August on foot of a European Arrest Warrant issued earlier this year by the High Court in Dublin.
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.
Mr Hutch was arrested in Spain in August and appealed his extradition.
He argued that it was "strange" that there had been no extradition request for five years from February 2016, that his life was under threat and that the Special Criminal Court was not a competent court to try these allegations.
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However, the three judges rejected his appeal against extradition but accepted his application to be allowed, if convicted, to serve his sentence in Spain.
The ruling stated: "The decision to extradite Gerard Hutch … must stand."
It also said: "If he is convicted, this man should be returned to Spain so he can complete his sentence here as he has residency status in Spain."
In his appeal, Mr Hutch claimed that he had decided to become a full-time resident in Spain because of the threats to his life and insisted he had legally changed his residency status and paid his taxes in Spain "without trying to hide".
The court accepted he was Spanish resident, that he had an FIE - a foreigner's number - had relevant working ties to Spain for a number of years and paid his taxes in his own name.
It is the second time Gerard Hutch has lost his appeal against extradition. The ruling also stated that this decision is final and cannot be appealed.
The decision was taken on 14 September but has only just been made public.
Mr Hutch is understood to have been informed by his defence lawyer in prison in Madrid.
Gardaí say they are aware of the ruling and that they are monitoring the situation.
They say they are liaising with the Spanish authorities but stress that at this stage proceedings remain a matter for them.