"A Depressing Curse on the Face of Scotland"

"From the beginning, Celtic was identified with working-class, Irish Catholic immigrants. Rangers soon came to represent a contrasting (and depending on your viewpoint, reactionary) vision of a Scottish identity that was conservative, middle-class, and Protestant. Matches between the two were frequently interrupted by pitch invasions and fighting. In 1909, a Scottish Cup final contested by Rangers and Celtic had the distinction of hosting what historian David Goldblatt calls the first 'full-scale football riot.' (It was a doozy—fans lit the stands on fire, and when the fire brigades showed up, the crowd hurled stones at them.)
"Though the sectarian conflict was partly based on economic anxieties, it was, ironically, enormously beneficial to the clubs, which took in huge gate receipts as supporters flocked to see the team with which they identified. The Old Firm soon dwarfed the rest of Scottish soccer—to date, Rangers and Celtic have won 95 of 114 league championships. If either club hadn't existed, the other would likely have lived out its years quietly—nothing more than a run-of-the-mill, non-politically-signifying soccer club. Because they became symbols for fans who wanted to kill each other, they're the greatest teams in Scotland."

Brian Phillips in Slate discusses the Celtic-Rangers soccer rivalry.