According to his mother, Margaret, Conor McGregor "was born with his fists clenched".
That fighting spirit - which was kindled on July 14, 1988, when he was born in Holles Street - has fuelled his rise from a trainee plumber on the dole to earning over €65m in a boxing bout against Floyd Mayweather in Las Vegas on Saturday.
The man himself has recalled a neighbourhood "synonymous with gang culture," but his father Tony paints a different picture of life in the semi-detached house in Clonmacnoise Road.
"We had great years living in Crumlin," he says. "Conor was born and spent his formative years there. There are some great people in Crumlin. You just have to be streetwise.
"Drugs have ravished most areas and that happened to Crumlin. That's still going on to this day. Overall, though, Crumlin is a nice, respectful, working class area."
There was nothing which marked young McGregor out as a future global fight star.
Margaret McGregor, mother of Conor, speaks with Lorenzo Fertitta, Chairman & CEO of UFC. Photo: Sportsfile
"It was a very normal upbringing. He didn't really want anything. He had all the latest toys, the latest PlayStation and the latest bike. There was nothing outstanding about his upbringing at all. That's what makes it all the more remarkable for us."
McGregor was 13 when Tony moved the family to a bigger house in the leafier surrounds of Lucan keen to provide a stable home for his young family. The move, though, meant a new area and a new school.
Though boxing was his first combat pursuit, it was not his first sport. As a child, he had followed Manchester United and, aged seven, with a mop of blond hair, was already kitted out in red and white. McGregor played football for Lourdes Celtic and other local clubs, but in his early teens he opened the door to Crumlin Boxing Club. He did not look back.
"I always knew he had capable skills as a boxer," explained his father. "My father was a boxer as well. He was a championship boxer, but only at amateur level."
McGregor would return again and again for three years to Crumlin Boxing Club, under Phil Sutcliffe, a two-time Olympian for Ireland.
"He was a good kid, he did as he was told," recalled Sutcliffe. "He trained harder than a lot of the other kids. He was extra careful of not getting whacked. He was a very cute orthodox southpaw, and a very good counter-puncher.
"He was just getting good when he went to that mixed martial arts stuff. I didn't like it. I still don't really like it. I don't like anybody being hit on the ground and no one coming in and them constantly being hit.
"Most of Conor's fights (in the UFC) are stopped with punches. He got his boxing ability from here. He punches the way he was taught - properly. That's probably why he's knocking so many guys out."
McGregor matches his brash deeds with words and the trash talk troubles Sutcliffe.
"People look up to him. I think he's only using the language because it draws more attention. That's not the Conor we knew. The Conor we knew tried very hard and was very dedicated. He knows what he wants. He draws attention. He has that quality. He's like Muhammad Ali in a way. A lot of people want to see Conor get a few clatters. A lot of people want to see him win."
But outside boxing, a new sport was emerging. For McGregor, MMA was fast, furious, different - and utterly captivating. "I think he knew where he was going and he was getting better results with the UFC," said Sutcliffe.
"He was getting better results there than he was with boxing. It was happening faster there and he could see something that was good for him. He's proven it."
McGregor's timing was in again when he joined a training group which included former UFC fighter Tom Egan, and martial arts coach John Kavanagh. They formed the perfect trinity, and McGregor had a new calling.
"He just had a love of mixed martial arts. It took over his teenage life," said his father.
"Conor had been popping in and out of different sports until he found something that suited him. That must have been tough on a little 14-year-old kid.
"It's kudos to Conor for being able to overcome that as a young boy. He was put in a different area, all his mates were left behind and he had to find a new school. He met Tom Egan in that school and Tom's love of mixed martial arts rubbed off on him. Once he found his niche, he knew then he was going to make it."
McGregor's wing man, Egan, recalls them being allies. "I believe it was fourth year, fifth year of high school or something like that; I remember seeing Conor," Egan recalled.
That led the pair into the path of Kavanagh and his Straight Blast Gym, although there were concerns when McGregor left school at 16.
"He never had an interest in taking education further," explained his father. "One of his teachers at secondary school said he had the intelligence to become a lawyer. That he had the academic ability to uncover and analyse detail. But he just didn't have the interest. He knew what he wanted to do. He was ahead of the curve. I didn't see a career in it at all. But I've thankfully been proved wrong."
The SBG training facility is now world-renowned. Paddy Hoolahan, a former UFC fighter and training partner at SBG, said: "Conor's like the commanding chief at the moment and John is the general. When Conor has won belts in the UFC, all the guys rolling around here get the chemicals that come from watching another Irishman get to the highest level and win a world title. That's enough motivation on both sides."
Hoolahan says McGregor and Kavanagh make the perfect duo. "Even though Conor truly believes in his ability, John is a sceptic. He has the mind and the reason. He will put a plan in order and guide things. It's a perfect combination."
There is also no change in him, either. "Conor's always been the same to me. He has suits and stuff but he deserves every single one of them. He sets trends. Conor is the biggest sporting athlete in Ireland. He's probably in the top five or top 10 in the world. He said it, he believed in it, he stuck it out. He deserves it."
As UFC president Dana White described this week, McGregor is "about to earn life-changing money", but it was tougher when he started out.
"By the time he'd won two titles in (European MMA organisation) Cage Warriors, we were 100 per cent behind him," said Tony McGregor. "I was still a bit concerned, though. He was getting probably €2,000 pay cheques, which wasn't bad for a kid his age."
Aged 20, McGregor had a life-changing moment when he met his heroes - UFC star Chuck Liddell and promoter White - during a UFC visit to Dublin for an event, in 2010.
McGregor was now fired up, ready to excel. He was even calling on the family's famed warrior lineage.
"The name McGregor is steeped into Celtic warrior folklore," McGregor once explained. "The Irish passion for fighting comes from a long history of battle. I know my ancestors and my bloodline is on the battlefield; they were doing it riding a horse with a sword, swinging an axe. Now I'm doing it in the Octagon."
With victory after victory in the UFC, predicting how and when he would win, McGregor's army of supporters grew rapidly.
Without the support of the McGregor family, the fighter may never have risen to such heights. They remain a close family. Tony and Margaret, his parents and his elder sisters, Aiofe, a self-employed make-up artist, and Erin, travel to all his fights.
"You will get some haters, but the majority love him for who he is. He's honest. He's very generous," said Margaret. "I think he gives a lot of people hope. He is everything you see, but he's still honest and says it how it is. I'm extremely proud of him. He is a mummy's boy. He'd probably kill me for saying that but he is. "
Erin goes even further.
"As an older sister to watch your younger brother succeed in his dreams, the word proud doesn't do it justice. I love the fact he stays true to himself. Even though he's my little brother I still hugely look up to him and admire him for that."
UFC President White now counts McGregor as one of the greatest stars of the fighting organisation's history, recently describing him as 'The Unicorn' with the fighter having held both featherweight and lightweight titles simultaneously, while consistently drawing a pay-per-view audience of over a million.
"Conor is not just good at the fighting and the talking, he's good at every type of warfare there is in the business," explained White. "The mental warfare, the physical warfare, the whole thing. That's why I call him The Unicorn.