i probably shouldn't but here goes ...
Some Celtic fans (quite a lot of Celtic fans) identify very strongly with their Irish heritage (loads of Irish folk moved to Glasgow and West Central Scotland in the 19th & 20th centuries to the extent where a significant proportion of the entire Scottish population can claim some Irish heritage - since the gravity well for the Scottish population centres on George Sq, Glasgow, and encompasses the surrounding conurbation that stretches off in all directions: Lanarkshire, Dunbartonshire, Renfrewshire, bits of Ayrshire etc)

A reactionary movement of Protestant Scots frowned on Irish immigration ("coming over here with your huge families, bizarre church, your potatophilia, taking our jobs and still subscribing to a religion run by the Antichrist in Rome which was a problem we *thought* we sorted out in the Reformation back in 1560...")

The invention of football in the late 19th century gave this antagonism a perfect platform (Celtic for the Irish identity, Rangers for the West Central Scotland Prods) and as matters developed in the 20th century (and especially after Northern Ireland kicked off in the late 1960s), Rangers were officially bigoted, particularly pro-Union (that's the 1707 Union of England and Scotland) and had a portrait photo of the Queen in the team dressing room. Outside the Celtic ground meanwhile, you could often buy the latest copy of Am Phoblacht (the Sinn Fein newspaper) ...

So although perceived sometimes as Celt v Celt, the Celtic-Rangers thing is much more an expression of British state v Irish republicanism (or Belfast Lite as Dara O'Briain sometimes describes it) ...

Speaking as an East Coast boy, my view is "a pox on both their houses" ...
( writerblock ... embedding disabled by request on, Sun 4 Apr 2010, 19:51, 1 reply)

Cheers
and I pretty much know all that, I like to read the Scottish football forum on the BBC when I'm bored. The one thing that unites the Old Firm is their hatred of the English apart from a few Rangers fans who wear England tops as some kind of ultimate wind-up of the Celtic fans though I guess they switch to Argentina/whoever is playing England come the World Cup.

Edit: furthermore, I can't say I'm an expert living in Hertfordshire and never having been to a game but I gather that there are more Celtic fans singing about the IRA outwith (I can even talk Scottish) the Old Firm games because security isn't as tight and that pretty much every other club has fans that sing Billy Boys when Celtic come to visit. So it's not just the Orange Order/Masons in Glasgow/Norn Iron.
( stopmeandslapme Cometh the hour, cometh your mum, Sun 4 Apr 2010, 20:40, closed)

...
"pretty much every other club has fans that sing Billy Boys when Celtic come to visit"

i used to have a season ticket at Pittodrie and I live near Easter Road: i can guarantee that you will never find people singing quasi-Rangers songs (Billy Boys etc) at these grounds ... can't say what happens elsewhere with cast iron certainty because i don't go and see Motherwell v Celtic, Falkirk v Celtic, Killie v Celtic ...
( writerblock ... embedding disabled by request on, Sun 4 Apr 2010, 22:55, closed)