It was Albert Einstein who stated that the most powerful force in nature is the power of compound interest. Transposed into the context of demographics, if a minority group 'A' has on average 5-6 children per family, and the majority group 'B' has just two, then it is just a question that of time, before 'A' inevitably becomes the majority. This is what is happening in NI.
In southern Ireland, the situation was different. The demise of the Protestant population which at the time of independence stood at circa 15% but quickly thereafter diminished to its current 4.25%. There were two main reasons for this: one being the return after Independence in 1921 of large numbers of the Anglo-Irish middle class to England, where they quietly re-integrated themselves into the country of their forebears. The other principal reason for the decline in the population % of southern Irish Protestants, was insistence by the Catholic church that in the case of mixed marriages, that the children be raised as catholics.