Whoever changed the title of this thread, seems to have forgotten two things.
1 It's a Protestant v Catholic thing
2 Ireland is not a wholly Roman Catholic country, even if you count it as 2 parts.

Glasgow Celtic was started as a team for Roman Catholic boys, and for many years no protestant ever played in the team.
The adoption of the Irish Tricolour, and the singing of Anti British songs, is a recent development in their long history.