I've heard the complaint about Celtic tops aired by lots of fans at every Ireland match I've ever been at - it's usually held in the same regard as Leprechaun beards and huge green plastic hammers. So whether people are 'clued-up' as you say is debatable.
Anyway regardless of historical ties or colours, Celtic are not actually an Irish club whereas St.Pats's are. Might as well wear Hibernian or Panathinaikos tops. Also, anecdotally, an LOI top wearer would be likely to at least wear an Ireland scarf or hat (if colour is the issue).
Call me old-fashioned, I just like people at games in regalia of the teams actually playing. Anyone who isn't is likely to be branded a part-timer, as they would anywhere else.
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