Easter 1916 Rising

[BrianSoddingBoru4]

By the same law as everyone occupying and annexing foreign soil as their own. The Germans may have had the power to annex the Alsace in 1871 and they may have kept it, if they hadn't lost WWI. It's just that the more powerful country can hold on to what it occupied. But that doesn't make it right by definition. It's still occupied territory without it's population having any say on the matter.

The case of Northern Ireland is far from that simple.

Correct, it isn't that simple.  But the 'legal' foundation for British (Norman at the time) possession of Ireland was the Laudabiliter, in which Adrian IV gave Ireland to Henry II.  Even if the Bull was genuine (and it probably wasn't, just a claim by the land-hungry Angevins), it wasn't legal, as Ireland wasn't Adrian's to give away.  Successive English kings continued to claim Ireland under Papal authority (again, not legal) until Henry VIII bullied Ireland into the COI Act.

In other words, English possession of Irish territory is essentially the result of a forgery, a theft, and a mugging.

Boru