We had loads of Irish flags before the orange kit came out...
On a serious note, most cities have a catholic team and a protestant team. Usually, the catholic team play in red and the protestant team in blue:
Catholic:
United
Liverpool
Celtic
Arsenal
Protestant:
City
Everton
Rangers
This could all be coincidence - I have no idea. Perhaps someone could enlighten me?
In any case, these ties are still pretty strong today even if the religious connection has been somewhat diluted.
eg when celtic play the scum, you will see hundreds of half scum, half celtic scarves. rangers and everton have close ties and lots of players have played for both clubs, eg Gazza. You couldn't imagine Gazza ever playing for Celtic, or Roy Keane for Rangers. I had the misfortune to attend the giggs testimonial scum vs celtic, and both sets of fans were chanting and had banners of 'keano', even though keane had not played for celtic. Odd.
eg when the catholic teams play, you see rafts of irish flags in the crowd. With everton, city and rangers you're more likely to see Union Jacks.