As we all know, all of the island of Ireland is still part of the United Kingdom of Great Britain and Ireland, after Home Rule was passed (and enacted) in 1914.
Why did the Union stay together? Why is stable today (the independence referendum back in 2010 failed by a margin of 38%-62%) despite the not-so-pretty history of secretarian conflict in Ulster?
What would have happened in an alternate reality where Ireland (or a major portion of it) left the Union?