One question we may ask of the Irish nationalists who live in Northern Ireland is: given that they claim to have a strong preference for living in the Republic of Ireland, why don't they? Rather than pushing for Irish unification politically, a cause which is no closer to success than it was 95 years ago, why don't they just move 50 miles south to live in the existing territory of the Republic? I shall consider various reasons they might have for not moving, and ultimately conclude that in general they just don't care that much. The preference of Northern Irish Catholics for Irish unification is not a preference that we should take especially seriously.
It is worth making it clear that I am not arguing that by living in Northern Ireland, Catholics consent to British rule. David Hume savaged consent theory quite comprehensively back in 1748. In any case, the idea that living in a state constitutes consent to that state presupposes that the state already has legitimate ownership of its territory. Nor would I claim that Northern Irish Catholics lack strong feelings about which state ought to possess sovereignty over Northern Ireland. But such feelings are produced by a need for group identity rather than any intellectual case or any experience of being oppressed.
The costs of moving to Eire
Let's be fair: there are substantial costs involved in moving house, especially between countries. But for most people in Northern Ireland, I shall show that this is not a convincing explanation. Most of the costs involved in such a move are small, negative, or inevitable.
Let us divide the costs into four categories: material costs, social costs, legal barriers, and transitional costs. By material costs I mean long-lasting reductions in one's standard of living as a result of moving geographically. An example of a material cost would be moving but being unable to find a job similar to the one you had back home, with the result that one is permanently poorer. These are the kind of costs that explain why people who are still in work do not tend to move from higher-income countries to lower-income countries. For much of the last century, this would have provided a plausible reason for not moving to the south: at the time of partition, Belfast was the only significant industrialised area in the island of Ireland, and most of the Republic was dirt-poor. But since around 1990 Ireland has undergone rapid economic growth, to the point where its GDP per capita is much higher not only than that of Northern Ireland, but of the UK as a whole. Nationalists moving to Ireland nowadays would most likely improve their standard of living.
Social costs are the long-term changes to one's social life that are necessitated by moving. These can exist in both losing old friends, and losing access to activities that one enjoyed but no longer has access to. Such costs can indeed be substantial - but they are not plausibly especially large for most Northern Irish Catholics contemplating a move south. They would not be moving far - Belfast and Dublin are only two hour's drive apart, absolutely fine for regular weekend visits home to see family and friends. The cultural life available to a Northern Irish Catholic is not tremendously different from that available to a citizen of the Republic of Ireland. If people really care, you might well persuade a lot of people to move south with you!
The legal barriers are close to non-existent. UK citizens born in Ireland are entitled to Irish citizenship, and do not have to give up their British citizenship to acquire it. The border is unguarded, indeed in most places unmarked. Perhaps there might be some problems for former IRA members, given that the Republic was generally quite successful in keeping the IRA out of Ireland. That said, I'd guess that since 1998 with the general amnesty available, this should not have been an issue. In any case, most Northern Irish Catholics were not members of the IRA.
Finally, the transitional costs. There are genuine costs to finding a new house and a new job, even if you are moving into a higher standard of living. But what proportion of Northern Irish Catholics have lived in the same house for all of the last twenty years? Perhaps members of the older generations have significant attachments and no reason to move beyond nationalist sentiment, but for any adult below the age of forty (and probably most above that age) they have surely had an opportunity to move to the Republic of Ireland at no permanent material cost, minimal social cost, with no legal barriers, and no transitional costs beyond those which they would have faced anyway in moving between two houses in Northern Ireland.
In sum, the revealed preference of Northern Irish Catholics is that they don't care all that much about whether they live in the UK or the Republic of Ireland. The overwhelming majority could have moved south at minimal cost, perhaps even at a gain, and have chosen not to do so.