Celtic for me Maroon, largely down to Striker & Sensible World of Soccer on the Amiga when I was a kid, plus large family ties in Ireland and Scotland who all hated Rangers (dunno why, they weren't Celtic fans!). Then there was one of my idols growing up, Henrik Laarson. Now it's because of a few friends who follow Celtic intensely, and I want them to be happy.
No disrespect intended, but I find Scottish football boring. Not just because of the domination if a small group of teams, but mostly the football - it's often riddled with Sunday pub team mistakes that bewilder!