Just a couple of points, Robert.
Hibernians were also founded by the Irish, yet there is no mad scramble over here to support them. Which leads one to assume that Celtic fans are mere glory hunters. Would they still be as supported if they were playing Falkirk and Stenhousemuir every week.
The other point is that over here, Celtic are an easy team to support. Their only rivals are Rangers, and not many people are going to walk around in a Rangers' shirt, so its safe enough. An "easy" option.
What bugs a lot of us though, is the fact that thousands of people over here support a Scottish team, when football grounds over here are three-quarters empty. I'm sure the same is true in Scotland - people from say, Stranraer or Dumfries will support Celtic or Rangers, rather than the local side. And, whatever about the quality of our league, its definitely more exciting than the highly predictable fare served up in Caledonia.
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