Hi Joe, Sorcha did a pretty good summary there.
What was missed out was that Glasgow Celtic are from an Irish Catholic background, Glasgow Rangers from Protestant.
These sectarian divisions have been with the clubs throughout their history.
For many years Rangers wouldn'd sign a Catholic player and Celtic vice versa.
At various times both clubs have made efforts to distance themselves from this problem, to varying degrees. I admit that Celtic may have made a greater effort in recent years.
Ghirls is new terminology to me, Bhoys however, refers to both the players and supporters of Celtic F.C. The supporters should be able to tell you the history of the term, I presume it's something to do with Irish pronounciation but I may well be wrong on that, I'm just guessing.