I suspect it is just where you live. Where I grew up, there were few Irish people and Rangers was almost universally supported. Of course, a lot of the Rangers heartlands are vastly populated by Irish, albeit Northern Irish Protestants.
If you name a district of Glasgow, it's pretty easy to assume allegiances - although I was surprised to see a large Orange Lodge and Rangers pub in Parkhead, bedecked in flags, when I was last down there. I don't think the other lot would get away with that in Ibrox or Govan.
But yes, I'm a rather inevitable bluenose. My old grandfather, bless him, would've countenanced nothing less when I was growing up.