In cases like N. Ireland and Kossovo the 'wealth= more fit" argument hardly worked as these (typically) poorer population subsets (Catholics, Albanians) eventually overwhelmed the wealthier ones.
Unless you've just stepped out of your time machine and have access to information the rest of us don't I think it is more than a little premature to claim that the Catholics in Northern Ireland have "overwhelmed" the Protestants. The Protestants are still in the majority (and are likely to be for some time) - this is why when there was a devolved assembly the First Minister was a Protestant. If it is ever restarted it will still be a Protestant, unfortunately this time it will be the monstrous Ian Paisley.
Or maybe you've just been watching ST:TNG
Confused ? You will be...