The Dublin man, named locally as Patrick Caslin, had been in the intensive care unit in James Connolly Hospital in Blanchardstown but died from his injuries this afternoon.
Mr Caslin was an associate of a notorious Finglas criminal - known as Mr Flashy - and he was due to go on trial next July at Dublin Circuit Court on six charges of money laundering related offences.
He had been arrested as part of an investigation into debt collection activities linked to the Finglas based organised crime gang headed up by Mr Flashy.
It is understood that Mr Caslin has been previously warned by gardai about an active threat against his life.
By the time of his death, Mr Caslin had no serious criminal convictions and only had minor convictions for road traffic offences.
Gardai are continuing to probe how Mr Caslin received his fatal injuries and a source has said officers are investigating if he accidently shot himself while transporting a gun.
It is suspected that the weapon - possibly a shotgun - accidently discharged while in his possession, shooting him in the head.
“It is not definite but the most likely theory at this stage is that this man shot himself most likely by accident.
"The weapon which is suspected of being a shotgun has not been recovered,” a source has said.
“No other suspects for the shooting have been identified,” the source added.
Gardai said on Thursday night that no firearm was recovered at the scene despite initial reports that a shotgun was found beside the severely injured man.
However it is understood that it is “very possible” that associates of the injured man may have grabbed the gun and fled from the scene before gardai arrived according to sources.
There were a number of searches carried out in the Finglas area but the weapon has not been located and there has been no arrests in the case.
Caslin’s close pal ‘Mr Flashy’ is becoming under increasing pressure with sources saying that many of his old comrades have “turned against him” including a number of major Finglas criminals who are currently serving lengthy jail terms for a Kinahan cartel feud murder plot.
The crime group have also been involved in a number of disputes including a feud with a Blanchardstown based gang while ‘Mr Flashy’ also has close links to the Kinahan cartel.
Earlier this week, gardai made an appeal for information relating to Mr Caslin's shooting.
“Gardaí in Finglas are investigating all the circumstances of the discovery of a male with serious head injuries on the evening of Thursday 11th November 2021,” a spokesman said.
“Shortly before 8pm, 11th November 2021, Gardaí were called to a green area beside Mellowes Avenue, Dublin 11 where a member of the public reported having discovered a male with serious injuries.
“The male had suffered serious head injuries and remains in a critical condition in Connolly Hospital, Blanchardstown.
“An Garda Síochána are appealing for any person who was in the vicinity of Mellowes Avenue, Mellowes Road, Kildondan Road or Cardiffsbridge Road between 7.30pm and 8.00pm on the 11th November 2021, particular any person or driver with dashcam or other video footage to contact investigating Gardaí at Finglas Garda Station 01 – 6667500, the Garda Confidential Line 1800 666 111 or any Garda station,” he explained.