The origins of at least 4 of our teams are directly linked to a period where Scotland was booming economically and looked to Ireland, both North and South for workers to work in the mines, shipyards, iron and steel and other industries. Like any immigrant group they formed close communities and in at least 4 cases this included forming football teams. These were Hibernian, Celtic and Dundee Hibernian (Now United) plus Rangers. The shipyards at Govan followed the Belfast model of Harland and Wolf and employed almost entirely a "Protestant" workforce, Rangers then became the club of choice for the latter group. Hibernian were the most closed of these clubs in the early days in terms of players who had to be church going catholic. The others, including Rangers in the early days, discriminated but had prominent players of other persuasions. The Turbulence of the First World War in which all of Ireland sent troops was followed by the Irish gaining independence and the partition of Northern Ireland as a mainly protestant state. The second world war exacerbated feelings as Eire chose, as was their right, to remain Neutral. During the period until the late 1960's and the start of the "Troubles" it was not unusual for newspapers to carry articles which were anti Irish. Employers (some Banks for example) did not employ Catholics. The two big Glasgow Clubs reflected the times in which they lived and the employment practices at Rangers reflected this. Arguments can be made that separate schooling helped underline the apparent divide between the groups but that is a complicated argument. Hibs and Dundee United moved on and left their Irish Identity alive but Not Front and Centre. Society helped create the Glasgow Clubs as they are. Emptying Glasgow of its population in the 60s,70s,80s meant that displaced people (around 500,000) took with them their "loyalties" throughout Scotlnd. Sectarianism is less of a factor now than in the past but has a generation or two to go to come close to eradication.
My view anyway - for what its worth,