Didn't the IFA cease selecting players from the Irish Free State/Republic for both British Home Championship games and FIFA competitions by 1952 though? (See paragraph 53 of the
Kearns judgment.) As far back as 1946, eligibility for FIFA competitions was governed by article 21, al. 2 of the Regulations of the FIFA, which read as follows:
The players (NB. of International Matches) must be selected by the National Associations concerned and be subjects of the country they represent.
This was made very clear to the IFA by Ivo Shricker, the then-general secretary of FIFA, in a letter to them dated the 17th of April, 1951.
I would imagine the rule in place during the 1970s was very much similar in effect, in that I don't see how non-British citizens might have been considered by the IFA to be eligible to play for the IFA. There was no reason for the FAI to fear the IFA (at least not on legal basis) or to think the IFA were free to do what they liked as the IFA had already been brought fully under the jurisdiction of FIFA by that point. Likewise, the FAI would have had nothing to fear in selecting McGeough as they wouldn't have been falling foul of any rule in doing so, but perhaps you're right in that they feared some IFA backlash or quasi-diplomatic incident even if it doesn't appear rational when viewed through the optic of the modern-day context. I wonder what ever became of the letter McGeough penned to FIFA... If FIFA had sanctioned his wish to play for the FAI, then what could the IFA have done about it? It's not as if he was tied to them or anything.