Say the family tree looks like this:
Great Grandfather <- Born in what is now Northern Ireland Grandmother <- Born in Britain Mother <- Born in Britain Son <- Born in Britain
From a cursory glance of the relevant legislation, the grandmother and mother would be eligible for Irish citizenship as a foreign born citizen but the Son would not.
However, if the Grandmother or mother were to choose to exercise their right and obtain their Irish citizenship, this would reduce the number of generations between the son and the last Irish citizen ancestor to less than two. Would the son then be able to claim Irish citizenship? Finally, does it make any difference if the Great Grandfather was born in what is now the North or the Republic?