At point of a gun or by the bombing a civilian population in acts of terrorism you can slice off part of a country, or part of an empire and create a new entity. What you cannot do at the point of a gun or by the use of bombs is force unification to create a new entity. The latter is what the "men of the gun" in Irish politics have been trying since 1914 when a tiny minority of them secretly approached the Germans to request assistance to achieve their goal. Had, as Keith A stated, the Irish nationalists waited until the end of the "Great War", the Home Rule Act would have been implemented and Ireland would have ended up as a united, independent sovereign state in 1931. Instead the "men of the gun" opted for war, first with the British, then with each other. History repeated itself when the Second World War came along and the IRA (An illegal organisation in Eire) forged links with the Nazis to try and united North and South (They failed - Dev introduced non-jury trials and internment to stop them), they tried again in 1950s border campaign and their "Volunteers" got the shock of their lives when they found out that the North would fight to maintain their ties with the UK, the campaign ended in total failure, apart for rather spectacular and well planned and executed raid early on, the "campaign" was a complete and utter farce. In the troubles the NI Human Rights Campaign had the total backing and support of the people in the UK, for once the "Official" IRA saw that and took the decision to stand to one side, a view not shared by some members who formed the "Provisional" IRA, so the "men of the gun" had another go at forced unification of Ireland - and again the failed spectacularly - the Republic of Ireland dropped it's constitutional territorial claim on the North.

Brexit might just achieve what decades of violence totally failed to - let's wait and see. As far as British Policy goes - they've been trying to get rid of Ireland since the mid 1800s. If the North finds that their interests are best served maintaining open trading links with the Republic then they can call for a referendum anytime they wish - no-one would stand in their way. Having had that referendum and for arguments sake we'll say that they opted to join the Republic, then a second referendum in the Republic would be held to agree to the North becoming part of a united Republic of Ireland. Interesting times.