The relationship between Celtic and Rangers started off as a friendly affair but then descended into sectarianism and, in Northern Ireland, the clubs remain totems of tribalism.

Five weeks ago a youth called Gerard Lawlor was murdered in Belfast. He was the fourth member of Saint Enda's GAA club in the north of the city to come to a violent end in the current resurgence of sectarian strife. He was wearing a Celtic football jersey and appears to have been killed because the shirt identified him as a Roman Catholic.

On Wednesday afternoon, the international football career of Neil Lennon came to an abrupt conclusion shortly before he was due to Captain Northern Ireland for the first time, on the occasion of his 42nd appearance for the team in a friendly against Cyprus at Windsor Park.

Someone claiming to be a member of the Loyalist Volunteer Force, but more likely a hoaxer, called the BBC in Belfast and issued a death threat against Lennon.

It was not the first time Lennon, a Catholic from Lurgan, has been targeted by extremists. In February 2001 he was jeered every time he touched the ball when Northern Ireland played Norway at Windsor Park and he was taken off prematurely because of the abuse. Lennon did not suffer from orchestrated catcalls - or death threats - when he appeared for Northern Ireland as a club player with Crewe Alexandra or Leicester City. But now Lennon, too, wears the green and white hoops of Celtic and that marks him, in the eyes of some, as a fully-fledged "Fenian bastard".

Celtic and Rangers, Glasgow's fierce football rivals, are as much products of the cultural exchange between Ireland and Scotland as St Columba, the Book of Kells, the tales of Finn and Ossian, and a Gaelic tongue. Whereas the rest represent shared spiritual and artistic values, the two clubs stand on the fault lines of the same process.

They cannot flourish without each other but it seems they are also shackled to a warped tradition they can't quite escape.

Celtic was founded in 1888 as a sports club whose main function was to aid the poor of the industrial east end of Glasgow, who were largely Catholic Irish immigrants, many from Donegal. Almost instantly, Celtic's football team became a powerful force on the field and the club became less concerned with putting bread in the mouths of the poor as giving those same mouths something to shout about. And shout they did, to the consternation of the native population, who cast about for a team capable of matching the raucous incomers.

Queen's Park were the obvious candidates, but although they were the oldest and most successful Scottish side, they were about to enter a precipitous decline caused by their attachment to amateurism just when the sport was about to sanction professional players. So the mantle fell upon Rangers. Celtic and Rangers got on famously for a while. When Celtic played their first game at the newly constructed Celtic Park, they invited Rangers to be their opponents and won 6-2 in what was said to be "a contest marked by appropriate light heartedness and good humour".

In 1892, Scottish Sport magazine reported that "the light blues are favourites with the Parkhead crowd". The following year the clubs travelled together to fixtures in Edinburgh and the same publication commented: "They are getting very pally. And why not?".

Why not indeed? Celtic and Rangers had discovered that their sporting rivalry was a profitable affair. They were soon suspected, with good reason, of contriving as many meetings with each other as possible. It was this notion of the pair as a business partnership that prompted another magazine, Scottish Referee, to print a cartoon in 1904 referring to them as "The Old Firm".

A century later, the nickname endures, but for most of the intervening period it could safely be said that the light blues were anathema to the Parkhead crowd and Celtic were loathed at Ibrox, although the directors of the two clubs periodically made common cause against the game's ruling authorities.

It is difficult to say exactly when bad blood between the two factions was established. Celtic had been, from the start, a club with an overwhelming Catholic Irish provenance while Rangers, founded by Highland brothers called McNeill, found favour with Presbyterian Scots.

It was natural that they should recruit from their respective constituencies, but no odium seems to have been attached to the few Protestant players who turned out for Celtic or the smattering of Catholics who were employed at Ibrox in the first couple of decades of their rivalry. In 1907, Willie Kivlichan, a Catholic doctor of medicine, played for Rangers while a student - and transferred to Celtic after graduation.

Politics seems to have been the defining ingredient in the developing tensions, rather than religion, although once the relationship descended into sectarianism it was all the same to the partisans. In 1912, the establishment of a major yard close to Ibrox by Belfast shipbuilders Harland & Wolff undoubtedly added an intransigent element to the Rangers support, comprised of incoming Northern Irish Protestant workers. Four years later the disproportionate losses sustained by the Ulster regiments at the Somme, combined with the aftermath of the Easter uprising, added to the divisions between immigrant Irish and native Scots, which by then were turning vicious.

The singing of rebel or loyalist anthems from both sides of the Irish divide was firmly established by the early 1920s, as were violent clashes between supporters. One phrase becomes depressingly repetitive in match reports for the next 50 years - "bottles were thrown". But the two boards could hear only the guineas clink as crowds swelled. The record attendance at any British league game remains that of the Rangers v Celtic New Year Derby at Ibrox in 1937, when at least 118,567 attended.

With a shift in Scottish public opinion from the mid-1980's onwards, as church attendances declined and corporate sponsors became careful about their image, Rangers abandoned their non-Catholic signing policy. When they won the League by beating Celtic at Parkhead three years ago, they were captained by an Italian Catholic and two of their three goals were scored by a Catholic from Greenock.

Both clubs have been enlisted to anti-sectarian campaigns and the signs are that there is a slow but discernable response from the fans. In Northern Ireland, however, the two clubs remain totems of tribalism and a beacon for extremists, as the examples of Gerard Lawlor and Neil Lennon illustrate all too clearly. In the North, it is not so much the wearing of the green which can be dangerous - but rather, whose green you wear.