Why is Northern Ireland significant in the Uniited Kingdom?

Because it was retained by the United Kingdom after the partitioning of Ireland. While the rest of Ireland is "the Repuiblic of Ireland", with own head of state, government and institutions, Northern Ireland is still part of the UK. This means Northern Ireland comes under the juristiction of the London based government and has no absolute power of self detirmation. Powers have been devolved to the Northern Ireland assembly so the people of the country can have more of a say in their own affairs. Northern Ireland also has members of the British Parliment, but nationalists (those who seek the unification of the whole of Ireland, seporate from the UK) wouldn't take a place there, as it would mean swearing allegence to the Queen, which would go against their beliefs. Some may aurge that the issue of Northern Ireland has been made less relavent through greater cross border cooperation between Northern and Southern Ireland, via both governments and the European Union. However there still are people on both sides who oppose either greater ties with the UK or greater ties with the ROI, so a devolved, power sharing executive (as is now) would seem to be the best current course of action.