It is claimed that you must attend an Old Firm game at least once in your life... like drinking yak's milk with Tibetan monks or fire-walking with Mayan Indians. To say that it is merely Irish Catholic Celtic vs Ulster Protestant Rangers is to disregard the centuries of cultural, ethnic and political nuances that make this one of the purest and most intense tribal rivalries that has ever existed in world sport. Today is the first of a handful of occasions this season on which these two will meet. For those new to this tumultuous carnevale it is a breathtaking experience. The seasoned observer learns a lingua franca with which to navigate this febrile part of Glasgow, when colours, songs, accents and dialects are scrutinised for signs of an allegiance to one team or other.

Even so, the series of events that befell Celtic's Northern Irish manager Neil Lennon between January and May this year remains shocking, and has led to one of those prolonged bouts of spiritual introspection in which Scotland likes to wallow every generation or so. Following the traditional early January meeting between these two clubs, in which Celtic triumphed at the home of their ancient foes, Lennon was told by police that he was the subject of a threat of physical violence and that his home in the West End of the city would be placed under 24-hour guard. Websites began to appear encouraging subscribers to "kill Neil Lennon"; bullets were sent to him at Celtic's stadium address. In March, following a four-day media blackout, it was revealed that primed explosive devices intended for Lennon and two other prominent west of Scotland Catholics had been intercepted.

Glasgow develops a bleak and sardonic humour that it uses as a carapace when evil comes to visit. But this time there was a palpable sense of shock that such things were happening in post-Christian, modern and liberal Scotland. Why Neil Lennon? In 2002, two years after joining the club as a player, he was forced to quit playing for Northern Ireland following death threats issued to him before a game. In 2003, two students attacked him as he enjoyed a night out near his home in the genteel West End. In 2004 he was the victim of a road-rage incident in the middle of the M8 which led to a man being fined £500. In 2008 he was beaten unconscious by two men as he left a pub. Each of his assailants was jailed.

If an asylum-seeker or an immigrant or a gay man had been subjected to an ordeal of such deep, sustained and violent hatred there would have been a public inquiry. Instead, there is a widespread notion that Lennon, somehow, has partly brought this upon himself. At the end of last month a 26-year-old man admitted to Edinburgh's sheriff court that he had assaulted Lennon during a football match in the city in May between Heart of Midlothian and Celtic. The miscreant could hardly have pleaded otherwise; 16,000 spectators and millions more watching on TV had witnessed the incident. The perpetrator spoke of his remorse and told the court that he had written a letter of apology to Lennon. The jury, however, found that the assault charge was not proven and that he had merely been guilty of a breach of the peace. It is one of the few occasions in Scottish legal history when an individual is acquitted of a crime that he freely admitted carrying out. A joke rapidly emerged that Colonel Gaddafi had offered to surrender to Nato if he could be guaranteed a trial in Edinburgh.

Until now, the Celtic manager has largely kept his own counsel on the maelstrom that engulfed him earlier this year. I encounter him behind his desk on the upper floor of the club's sleek training facility, set among the splendour of the Campsie Hills 10 miles north of the city centre. He is still in his dark tracksuit after a two-hour session with his squad and at 5ft 8in his squat frame and cropped red hair suggest an amateur boxing trainer. His words are considered and each sentence is weighed before he delivers it in a soft Ulster accent. He has an unnerving habit of adopting a longish pause before answering each question and looking directly at you, as if he is asking if you can be trusted not to distort or misrepresent. He seems at ease with himself and glad that he has made it unscathed to the start of a new season. But he remains angry and not a little bitter at what he had to endure a few months ago.

"This isn't something that just suddenly happened this year," he says. "When I joined Celtic as a player 10 years ago I became aware immediately that I was being singled out for special treatment. At first I thought it might just have had something to do with the fact that I was an expensive signing from the English Premiership. That would have been OK. But I became aware of something more sinister when I played for Northern Ireland as a Celtic player for the first time. Before then I had represented my country 36 or 37 times and had enjoyed the full support of our fans. Now I was aware of being jeered by our own supporters every time I touched the ball.

In the line of fire: Lennon is attacked by a Heart of Midlothian fan at Tynecastle Stadium in May. Photograph: Lynne Cameron/PA

"Then I also began to become aware that I was being jeered at every away ground Celtic played at. Some writers said that it was because of my combative style, but I only received two red cards in seven years playing with Celtic and I had never received this treatment while playing in England for three different clubs."

Lennon is a passionate man who, only with some difficulty, masks his emotions. When Lennon is angry he seems to snarl and bare his teeth. Sky's ubiquitous TV cameras captured just this look in the aftermath of a replayed Old Firm cup-tie in March. A lot had been riding on the match; Celtic had developed the upper hand in recent Old Firm clashes and Rangers were under pressure from their supporters to win. It wasn't the roughest game these two have played, but after a narrow Celtic win, eight Rangers players had been cautioned and three sent off. At the end of the game the cameras closed in on Lennon and his Rangers counterpart, Ally McCoist, about to greet each other at the final whistle. In the space of five seconds, the following happened in chronological order: Lennon shakes McCoist's hand and smiles in commiseration; there is a brusque exchange and Lennon adopts his Rumpelstiltskin visage; a mild scuffle lasting a moment ensues before each is huckled away. No more, no less. Within 24 hours Strathclyde's chief constable has phoned Alex Salmond, Scotland's first minister, and something called a "sectarian summit" of concerned politicians and officers of each club has been organised. It took almost as little time for Cobra to be convened after the al-Qaida attack on Glasgow Airport in 2007.

"I was proud of my team that night," Lennon says. "Almost every one of our opponents was booked or sent off. After the game I took exception to something that was said and a very brief scuffle took place. It was nothing." But it wasn't nothing and some pointed to Lennon's manner and said that this sort of behaviour… Well, he just doesn't do himself any favours.

Does this though adequately explain why so many people apparently want to harm him? After all, Lennon's former boss at Celtic, Martin O'Neill, comes from the same Northern Ireland, Catholic background as him and never encountered anything remotely comparable. "Unlike Martin, I played for a Celtic team that enjoyed seven years of almost unbroken supremacy over Rangers, so I was already an unpopular figure," he says.

In the 123 years since this fixture was first played there have been sporadic alarums to exercise the civic panjandrums. On these occasions there is an unseemly dash to occupy the moral high ground and speak portentously of Scotland's dirty little secret.

What had been a keenly contested but otherwise unremarkable city derby for around 25 years developed its sectarian edge when Celtic, the pride of the Irish diaspora, dominated Scottish football in the decade before the Great War. Who could mount a Scottish challenge to navvies, Fenians and tattie-howkers? At this time Rangers, whose home lay in Govan on the other side of the city, began to attract support from shipyard workers imported from the Harland and Wolff yards in Belfast. These were loyal to Ulster, the crown and the Protestant faith. Rangers became their club and began to shoulder their aspirations. Battle was joined. The two clubs, ever since, have been entwined in a grim dance to the music of their time.

Lennon was bemused at the reaction of Scotland's political leaders in the aftermath of the March cup-tie. "I still find it difficult to fathom how, in the face of death threats, attempts on my life and physical assaults, some commentators have said it is partly my fault. Martin O'Neill, my old boss at Celtic and also from Northern Ireland, once pulled me across to salute the Celtic fans at Ibrox after I had received even more than the usual abuse from the Rangers support. The press criticised him for that. Only one sportswriter supported him. This is 2011 and reporters have a duty to report what has happened to me in my 10 years here truthfully, but until the bomb threats they and the politicians swept it under the carpet."

"Yet I know that the Celtic support is very protective of me," Lennon continues, "and I am very humbled by that. I've had hundreds of letters too from Rangers fans, from Hearts fans, from Aberdeen fans, all saying that they have been outraged by some of the abuse directed at me and that it doesn't truly reflect their views. That's also been very humbling."

"I still find it difficult to fathom how, in the face of death threats and assaults, commentators have said it is partly my fault": Neil Lennon, playing for Celtic in 2006. Photograph: Andrew Milligan/PA

Lennon grew up in Lurgan, the sober and tidy little town that welcomes us to Northern Ireland when we leave the ferry, and where his parents still live. "I had a good upbringing and a good education," he says, "and I encountered very little sectarian tension. I played sports for many of the protestant boys' clubs and youth clubs. Sectarianism simply was not part of my upbringing." He was invited to train with Rangers when he was 13 and beginning to attract attention from clubs in the mainland. "I still have the letter Rangers sent to me," he says, "and a picture of me at Ibrox with Jimmy Nicholl [Rangers' Northern Irish defender]."

A friend, knowing that I am to interview him, speaks of her appreciation that he wrote, in his 2006 memoir, of his continuing struggle with clinical depression. His candidness was applauded by mental-health charities and he still does what he can to support their campaigns. Despite his troubles in Glasgow he loves the city, and people are often surprised to see him socialising freely in the streets around his home with his family. His friends say he is a gregarious and engaging character who will never be deterred from talking about football to anyone who approaches him.

The football writer Graham Spiers has been observing Lennon the player and Lennon the manager for 10 years. "Look, I'm a huge admirer of this man," he said. "But I wouldn't dismiss the notion that he brings some of his troubles down upon his own head. Yes, he does tick every box in the Celtic, Catholic Irish narrative and that elicits a bigoted response from some people. But this only explains part of it. He is always on the front foot and he never retreats when others have found it more prudent to do so. I've seen him give the middle finger to supporters who were giving him abuse at a couple of grounds. I'm sure these are occasions he regrets. At these times, he seems to be saying: 'Get it right up the lot of you.'"

But little of this begins to truly explain why, following 10 tranquil years in England, many people, outwith Scotland's Catholic community, simply can't abide the sight of this turbulent Irishman. The answer is both troubling and reassuring, for Lennon's decade-long Gethsemane may yet prove to be cathartic for a proud and disputatious nation.

The two other intended recipients of the explosive devices sent in March were Paul McBride, Scotland's most successful QC, and Trish Godman, the former Labour MSP for West Renfrewshire. The law and politics were, not so long ago, realms where the Catholic Irish were not permitted to hold a stake. The white and aspirational working-class Protestant community could rely on exclusive entry to the trades and the professions; the Kirk and the Conservative party gave them sustenance. Membership of golf clubs and tennis clubs and bowling clubs was theirs and theirs only.

That was then. No longer is there an artisan class in Scotland, once the sole preserve of working-class Protestants. And Conservative Unionism is extinct. The Church of Scotland is permitting gay ministers. Old certainties are now no more than wraiths and spectres. The Catholic Irish, having educated themselves out of the ghetto, are prominent in the law, politics, academia and journalism. And into this has come a ginger-headed, Irish Catholic captain and manager of Celtic FC. He is bellicose when once he would have been benign; he gets up when once he would have stayed down; he rolls with the punches and comes out fighting. There are some in Scotland who are raging against the dying of the light and Lennon has become a lightning rod for their fury.

In one of those city centre wine bars where tribal tensions dissipate in sauvignon blanc a well-known Glasgow lawyer is considering the Neil Lennon question. Douglas Kilpatrick is a lifelong Rangers aficionado and is uncomfortable with the easy assumptions about the opprobrium that surrounds the Celtic manager. "To some in my tradition, Lennon represents the sum of all fears, and religious prejudice on its own does not explain all of this," says Kilpatrick. "The influence and sense of entitlement which has sustained many working-class and middle-class Protestants have declined in the last decade or so. Some Scots just can't cope with this all at once and their response is suitably atavistic. But it will pass. There is too much healthy respect and goodwill between the majority on either side for it not to."

Lennon has refused to yield in the face of almost unbearable pressure and continues to live in the heart of the city with his partner and child while other Old Firm players seek the refuge of drowsy dormitory villages. Every man, though, has his breaking point and Lennon knows his. "If I thought for a minute that anyone in my family was in danger because of me then I would walk away tomorrow."

Perhaps Neil Lennon was born for this; to be the necessary paschal sacrifice that has to happen to wash away the sins of an ancient blood feud, which may now be coming, kicking and screaming, to an end.

• The following correction was published on25 September 2011:

Celtic's manager Neil Lennon was indeed brought up in Lurgan, but it is not "the sober and tidy little town that welcomes us to Northern Ireland as we leave the ferry". We were thinking of Larne. ("Why do people want to kill Neil Lennon?", Magazine).