From the viewpoint of either Wilson Principles (the British accepted Wilson's 14 on paper ) or Lenin's self-determination, The Irish are one nation and both the North and the South are Irish. Dividing them according to different voting choices was clearly British policy to continue their hold on the island. Ireland is one nation and the majority voted for independence way, it doesn't matter if Sinn Fein was majority in the North or not. The British has no right to classify the Irish as Southerners or Northerners. But we know it is just a tool for British foreign policy; if religious differences benefit UK, then they divide according to religious lines, if ethnicities benefit UK, then they divide according to ethnic groups especially citing the importance of national self-determination and if nothing benefits UK, then they just draw borders with a ruler just like in Middle East. If UK was so keen on protecting the rights of Protestant minority of Ireland, they could always had some kind of guarantee agreement with Irish republic.
Ireland may not be official state back then, who cares ? ESTABLISHMENT always changes. Irish people own that island and they manifested their choice in 1918 Election. I wouldn't care even if it was not offical UK election but just a poll with high voting ratio from all parts of the country and reliable results. Important thing would be to know the will of Irish people. Should the Irish people seek approval of UK Parliament to conduct war of independence so as to be seen as a legitimate movement ? Legitimacy comes from people, not from London.
Right, so let me get this straight.
Ireland is one and indivisible because you arbitrarily claim that the Irish are one nation and one people.
This Ireland has the right to self-determination because of Wilson's fifth point, and that the 1918 Election was a de facto expression of Ireland's will to become independence.
All tensions in Ireland, be they religious, ethnic or class-based, are all because of British foreign policy.
Government policy should be directed on the basis of opinion polls, because that is how sensible, stable policy is formulated.
Legitimacy comes from people, and not from London.
The people of Northern Ireland have no right to self determination whatsoever because Ireland is one and indivisible.
Northern Ireland does not have the right to self determination according to Wilson's fifth point: in spite of having its own separate language, culture and religion from Southern Ireland, and voting in favour of remaining in the UK, even though 1918 was a de facto referendum on the issue according to yourself.
Legitimacy comes from Dublin, not the people of Northern Ireland.