If he had obtained a passport, he would have been deemed an Irish national, as far as my understanding of Irish nationality prior to the amendments brought about by the Good Friday Agreement goes and whatever laws might have applied at the time. Whether he would have been bothered to, or even wanted to, is another matter; probably not. Anyway, the FAI might well have refused him in the extremely unlikely chance that he did volunteer, considering the prior agreement with the IFA from 1950 not to call up northern-born players.
However, as I said, it would have been very unlikely he would have been even remotely interested in playing for us considering Northern Ireland played in the somewhat prestigious British Home Championship at the time on a yearly basis and were seen as the better side anyway. The idea of northern-born Irish nationals representing us did not really enter the wider equation or players' consideration until the 1990s when the perception of the two sides' relative strength shifted in favour of us and, possibly, mindsets began to develop in favour of better acknowledging "individual identity rights" in the north as negotiations for the Good Friday Agreement commenced. It's important to be aware that the GFA didn't change anything statutorily as far as FIFA rules were concerned, contrary to common misunderstanding, because numerous northern-born Irish nationals played for us in the years prior to its passing. Personally, I just think mindsets changed, but I'm open to correction.