A Kinahan organised crime group leader is opposing his extradition to the UK to face trial on alleged firearms offences.
Liam Byrne was arrested in Majorca in Spain on Sunday night while he was having a meal with family members.
The National Courts Office in Spain confirmed that the 42-year-old is not agreeing to his extradition.
Byrne has been described in the High Court as heavily involved in drug trafficking and violent crime and a leader of the Kinahan organised crime group in Dublin.
Gardaí said they are not surprised at his decision to contest his extradition to the UK.
Byrne had been under surveillance since he flew into Palma Airport from Dubai on 26 May.
He was arrested and detained following an intelligence-led investigation by the UK National Crime Agency, which was supported by Spanish police and gardaí.
The Policía Nacional alleged that Mr Byrne's gang was "an arms trafficking organisation ... engaged in buying numerous pistols and sub-machine guns".
It said it was so significant in scale that one of its orders was "for 14 firearms in a single batch".
It also said that many of the firearms were allegedly fitted with "systems that did not leave fingerprints".
A second man, Jack Kavanagh from Tamorth in the UK, was also arrested in connection with the investigation.
The 22-year-old is the son of Mr Byrne's brother-in-law Thomas 'Bomber' Kavanagh, who is serving 21 years in the UK for drugs and money-laundering offences.
The NCA said it obtained Trade and Co-operation Agreement (TACA) warrants after EncroChat messages showed the two men were believed to be involved in the supply and acquisition of firearms.
Both men remain in custody and face further court hearings as extradition proceedings continue.
An official at the Audiencia Nacional, which is the national court based in Madrid, said: "The Irish national arrested in Majorca on Sunday was handed over to the courts yesterday.
"He was provisionally remanded in prison. He is not agreeing to be extradited."