How a displaced population could reconstitute Ireland

What would Britain do if the population of the Republic of Ireland mysteriously disappeared?

The answer depends on the period, but today the Irish diaspora would probably return to rebuild the state with British assistance.

The response depends on the period in which the disappearance occurs. Before the Second World War, Britain might quickly move into the empty territory. In a present-day scenario, the large Irish community in Britain and the wider diaspora would instead provide people able to return and reconstitute the state. Britain would probably assist that effort, making repopulation and institutional recovery the central problem.