One question Northern Ireland's geography raises, is why those parts of Ireland were kept when Ireland was given its independence in 1921 [see earlier post]?
The simple fact is Northern Ireland was separated from Ireland because of its Scot-English majority. The basic rule of thumb was to get majority Scot-English areas in the United Kingdom and majority Irish areas in Ireland. On the map the majority Protestant areas give a basic outline of the territory.
Factors involved in the green areas on the map, where Irish Catholics are in the majority, are several. 1. Islands of Ireland within Northern Ireland would not be strategically acceptable. 2. The big sort of thumb up the center would also raise strategic problems. 3. The areas along the southern border have near Scot-English Protestant majorities and have significant ownership of land and business by that group. 4. Urban areas, like Derry/Londonderry were too valued to cede.
We should note that getting ethnic peoples in their proper area, nation, country was a major theme in post World War I Europe. Making sure the Germans were in Germany and Italians in Italy, etc., was part of the peace process. committees in the process went down to the farmstead level to ensure people were in the right nation. That this cannot be done perfectly is one of the issues leading to World War II. Hitler saw Germans in ethnic areas outside Germany as deprived and bringing them into the German state was a major goal. The Sudetenland is a prime example of this concept being used. Especially islands of another group, or minorities of another group, became a battle cry.