Oisín Fagan parks his clapped out, two-seater jeep just off the Falls Road. "Don't even tell me what Amir Khan drives," he says with a grin. The 'fighting-Irish school teacher', as he is known, will take on the golden boy of British boxing at London's ExCel Arena on Saturday. The contrast between the two couldn't be greater.

It's not just that Khan, aged 21, drives a customised BMW M6 convertible and owns a mansion, while Fagan, at 34, still lives at home with his parents in Dublin. Nor that Khan blew £100,000 on a diamond-studded watch last year, while Fagan worries about paying a possible parking ticket. It's that if Amir Khan is all hype, then Oisín Fagan is all heart.

You'd struggle to meet a more genuine guy. He cares deeply about his fans, many of whom are working-class lads he grew up with in Tallaght. He even checked out the price of flights to London for the fight and released a message to supporters saying he reckoned Aer Lingus was cheaper than Ryanair.

Oddly, for any sports person, he believes in something other than himself. His conversation is peppered with talk about "standing up against injustice" and fighting inequality. "I'm proud to be a socialist," he says. Until July, he taught PE to the children of impoverished Mexican immigrants in inner-city Oklahoma.

It's Fagan's first one-to-one interview with a journalist. We're at the Royal Victoria Hospital where he's having routine pre-fight blood tests. While Khan is training in the Hollywood Hills, Fagan is working out of John Breen's gym, located on the floor above the Monico Bar in downtown Belfast.

You have to climb over a wooden pallet to get through the door and lumps of coal litter the stairs. "But the sparring is the best in Ireland," Fagan says. Breen has trained five world champions. Tinseltown wouldn't be for Fagan anyway. "Training in a warm climate is an advantage because it prepares you for the heat of the ring. But there might be too many celebrity distractions there for Khan. I'm very happy in Belfast." He wonders if Khan's lifestyle has cost him his edge. "He's got his money made and sometimes that means you lose the hunger. For me, this fight is the opportunity of a lifetime, it's my last big chance."

His family moved from Tallaght to Portmarnock when he was 12. Fagan studied physical education and political journalism at Oklahoma University of Science and Arts (he'd won a football scholarship there). But, since returning from the US, he has been unable to find a teaching job in Ireland. "The Department of Education are very snobbish. They look down their noses on American degrees. I understand that attitude with academic subjects but not with sport. How many Olympic medals did Ireland win compared to the US?" he asks.

In Oklahoma, he earned a reputation as a political radical. "I was captain of the university football team. After one match, I suggested we go to a bar. Two Kenyan teammates were refused entry. It was obvious they weren't getting in because they were black. I put up posters all over town urging a boycott of the premises.

"It got me into trouble with the college authorities. My scholarship money was cut. But I was brought up to do what I believe is right, whether it brings me trouble or not. To me, that's what being Irish is about. We've had to struggle for our rights ourselves. I don't view the term 'the fighting Irish' as derogatory – I see it as a compliment. It's our duty to stand up for the underdog."

And that's what he will be on Saturday night. Khan is the Commonwealth lightweight champion; Fagan holds the Oklahoma belt. They say that he is too old, too short, and too slow to worry Khan. He is just a meal-ticket opponent on Khan's road to restoring his reputation following his shock defeat to previously-unknown Colombian Breidis Prescott in September.

"I'm not surprised I'm being written off. The British always arrogantly hype up their boxers. Their guys are the best, that's it. They haven't done their homework on me. They haven't seen me fight. There are no tapes of me, nothing on YouTube, and that's to my advantage. I'm not the flash boxer Amir Khan is but I've loads of heart and passion."

Khan's record is far more impressive: 18 wins (14 knockouts) from 19 fights. Fagan's stands at 22 wins (13 knockouts) from 27 fights. Yet Fagan sees his strengths as his toughness and fitness. "I'd be the first to admit, I'm not the most stylish. I'm a fighter, not a boxer. I'll leave the pretty stuff to Khan. He'll know he's been in a fight by the end of it. I truly believe I can win. It's not Khan's fault he's over-hyped. He's a nice guy, by all accounts. I've no animosity towards him."

Khan is not a huge puncher but his speed is awesome. "He's the fastest thing I've seen since Floyd Mayweather," Fagan says. "I'll need to get in close to put the pressure on him. I'll have to make it an Oisín Fagan fight because if I give him too much respect and stay off him, he'll box the ears off me. I have to go for it but that's not a problem – I wouldn't know how to fight any other way."

Khan has already announced he is seeking a rematch with Prescott. "He's completely overlooked me," says Fagan. "If he thinks I'll be a pushover, he's crazy." Prescott's 54 second knockout of Khan reinforced the view that he has a glass chin. Khan's Cuban coach was sacked and he is now trained by Freddie Roach.

"I'm sure he's learned new tricks under Roach," says Fagan. "But you can't put muscles on a chin. If you're chinny, you're chinny – it's as simple as that. You can try to strengthen the muscles around your neck but it doesn't do much good. I think the Prescott fight will have affected Khan mentally. When he steps into the ring with me, it'll be ticking away in the back of his head."

Still, at 13 years younger and five inches taller, Khan's advantages are obvious. Fagan has gone 12 rounds only once in his career. But he denies he needs to knockout Khan early if he's to stand a chance. "Khan's camp might be saying, 'Look at the age of that fellow? He'll be no threat in the later rounds'. If that's what they believe, fine, but they're wrong."

Some of his big-name losses have been close calls. There was a points' defeat to Julio Cesar Chavez jnr. Last year, he lost a split decision to former IBF lightweight champion, Paul Spadafora, which many respected commentators think he should have won. There's no doubting his courage. Against Isaac Mendoza, he suffered a broken nose and cheekbone in three places, but he refused to have the fight stopped.

With only 10 seconds left, the referee called a halt due to Fagan's heavy blood loss.

He recalls his "first real scrap", aged 10, with two Traveller brothers in Tallaght. "They were stoning us. They were only kids as well but everybody was terrified of them. I took them both on. I got the best of it but I still went home covered in bruises and scratches. They were tough boys, fair play to them!"

He was no model child himself: "I was bananas, and I got my just desserts. I was kicked out of school just before the Leaving Cert for squirting milk at a teacher from a water pistol. It was kind of funny when I became a teacher myself." His own teaching style is strongly liberal. "A teacher has the power to make a kid's day horrible or great. I try to make it great. A lot of kids in Oklahoma had hard home lives. When they're in class in the morning, you don't know what they've been through the night before."

The chance to go to college in the US was a godsend for Fagan but there were still tough times. "When my scholarship money was cut because of my political activity, I struggled financially. I'd no health insurance and I ended up with medical bills I couldn't pay.

"After graduation, I had only $22 in my bank account. I was sleeping on friends' floors. My visa had run out, I had no job, and I faced deportation.

"Soccer was always my first love but, in Dublin, I'd used boxing to keep fit and train for football. So I went to the Badlands Gym in Oklahoma and asked them to get me a fight so I could raise the money to buy a plane ticket home. That's how it all started. And a school principal was at my first fight. He heard I was a qualified teacher and offered me a job."

Before he secured employment, Fagan had needed a handout from the Catholic Worker House charity. "When I got on my feet, I became a volunteer with them to give something back. We delivered food parcels to the poor. There were heart-breaking stories. I remember visiting one mother and her young son. They were living in a cheap hotel. She was very sick. When I walked into the room the smell of gangrene was over-powering. I wanted to do more than leave food. I took what money I had in my pocket and said to her, 'Look, the Catholic Worker House have a monthly draw for people they deliver food to. This month, you won. Here's your money.'"

Despite his compassion, there is nothing sanctimonious about Fagan. He's certainly not from the Barry McGuigan school of political inoffensiveness. His beliefs are hard-hitting. He lists his dislikes as "the British occupation of the North, smoking, scumbag drug-dealers, racists, snobs, bullies, imperialists, colonialism, corrupt politicians, corporations and insurance companies". Although he has no family ties with the North, his republican sympathies are obvious. His favourite movies include In the Name of the Father, Bloody Sunday and Some Mother's Son.

He laughs when told that ex-UDA commander, Johnny 'Mad Dog' Adair, is a big Khan fan. Adair was smitten when, in his prison cell, he first watched the Bolton boxer win silver in the 2004 Olympics. He has been at every Khan fight since his release. "Sure if Johnny Adair was cheering me, I'd be worried," Fagan jokes. He's marrying a young black dance teacher from Indiana in July. They plan to settle in Ireland. Indeed, he enjoys living in Belfast so much that he's considering a permanent move there.

Paddy Power make Khan 1-25 to win but Fagan's trainer, John Breen, says his fighter shouldn't be written off. "Khan's got the class but Oisín's got the determination." Fagan has been sparring with Paul McCloskey from Dungiven, the unbeaten IBF international light-welterweight title holder who is tipped as a future Irish world champion. "Paul gets the better of Oisín in the early rounds but, at the end, Oisín comes through even though he's half a stone lighter," says Breen.

Fagan still has no sponsor for Saturday's fight. An appeal in the Fingal Independent, accompanied by his email address, brought a few tentative offers from small local businesses but nothing concrete. "In the lead-up to Saturday, I'll need to be concentrating on the fight, not trying to deal with sponsorship," he says. "I'm not bitter, but sometimes I wonder how different my career might have been with a big sponsor or promoter, like Frank Warren, behind me."

Khan has a huge fan base but a hostile crowd at the ExCel Arena doesn't worry Fagan. "I've been there before. When I fought Chavez in Las Vegas, the place was full of Mexican lads."

Leaving the Royal Victoria Hospital, we meet a Falls Road couple who shake Fagan's hand when they hear he's fighting Khan. "Are you a celebrity too?" the woman asks. "Absolutely not!" he laughs.

sbreen@tribune.ie