A 45-year-old man who was arrested and questioned in relation to a fatal assault has regularly coached UFC champion Conor McGregor, the Herald can reveal.
Tallaght man Jonathan Dargan, who has trained McGregor in taekwondo, also works as a security man at the prestigious Lillie's Bordello nightclub.
He was arrested by gardai investigating the death of tragic Patrick 'Paddy' Mullally (57) over the weekend and released from Terenure Garda Station late on Sunday night.
Mr Mullally died after he was allegedly struck when he attempted to intervene in an argument between a couple following his own retirement party in the early hours of Saturday morning.
The shocking and tragic incident happened before 4am on Saturday outside the Park View Court Apartments in Harold's Cross, south Dublin.
Mr Dargan, who is originally from the Killinarden area of Tallaght, was not previously known to gardai until his arrest by investigating detectives on Sunday morning shortly after Mr Mullally died at St James's Hospital from serious head injuries.
Mr Dargan was questioned for a number of hours at Terenure Garda Station before being released without charge.
A file on the case will now be prepared for the DPP.
Mr Dargan has worked as a senior doorman at the well known Lillie's Bordello nightclub in Dublin city centre for almost two decades.
Sources have revealed that he has an "unblemished professional record" in relation to his employment as a nightclub doorman and was never in trouble with the law before.
It is understood he has been "distressed" about the death of a woman known to him.
Not known as a regular drinker, it is believed Mr Dargan had been socialising on Friday night and ended up in a verbal row with a woman known to him in Harold's Cross. It is understood that Mr Mullally attempted to intervene in this argument and ended up being struck.
Mr Mullally (57) had worked at the Guinness brewery in the capital for over 30 years and was returning home from his retirement party in the city centre. Friends described how the victim had stopped in his regular pub, Peggy Kelly's, on the way home.
He left the pub shortly after 3am on Saturday and, as he waited for a taxi, became aware of a dispute between a couple outside the nearby Park View Court apartment complex.
Tragically, as the Good Samaritan attempted to defuse the situation, he was struck and fell to the ground. However, he hit his head off a concrete curb during the fall, which resulted in him suffering severe head injuries. Emergency services were called and paramedics rushed to the location as the couple left the scene in the direction of Terenure.
An ambulance transferred Mr Mullally to St James's Hospital, but he was pronounced dead in the early hours of Sunday morning. No-one from Lillie's Bordello was available to comment when contacted by the Herald about Mr Dargan.
Friends who arrived at the scene of the tragedy over the weekend described Mr Mullally as a "gentle giant".
"It is just absolutely tragic, he was such a great man. Always in great form, always friendly, it's just devastating," said Pauric Keenan, a friend of the deceased, who had been with him on the night he passed away.
"We were only in Peggy Kelly's with him on Friday, we left around an hour beforehand. To think something like this could happen to a man like Paddy is shocking," Mr Keenan added.