Assuming no Great War, Home Rule is implemented with the Irish Volunteers forming the core of the Military.
The Northern Unionists attempt a 'July Rising', seizing Belfast City Hall, and several other sites in the City (Customs House, Anderson & McCauley Building, etc), in the expectation of 'Mainland' support.
When push comes to shove, however, what sympathy there was evaporates in the face of such a 'Knife in the back'.
'Irish' forces, with probable Royal Navy support move in and mercilessly crush the 'Rebels', causing significant damage to the City Centre.
In the absence of a Great War, political leadership is a little more far-sighted and few (if any) of the captured 'Rebels' are executed for their act of Treason.
Long prison sentences (Perhaps somewhere in Wales?) are the order of the day and an uneasy peace settles over the new State.
Chastened by the experience, Unionist Politicians focus on playing Kingmaker in the new Parliament. Their position greatly strengthened by the industrial dominance of their Heartlands.
Falkenburg