I don't understand why the UK is still holding onto Northern Ireland (speaking from an outside American point of view).
The situation in Northern Ireland is far more complicated than the stereotype "evil British oppressing the Irish" which your use of "holding onto" implies. Roughly half the population of Northern Ireland wants to be part of the UK and half of it doesn't for a very long and complicated series of reasons.
The British and Irish governments, working together, have been trying for decades to find a solution, which it now seems they have done with the end of terrorism, reduction of the Army and implementation of the power-sharing executive.