So I'm having another look at Albion's Seed. Fischer is mainly looking at the movement from 1717-1775. He'll actually cover a broader timespan than that.
His group is quite broad. While he says he means the borders (either side of the Tweed River), he includes all the highlands and Northern Ireland. He notes that they settle all over European North America but his focus is on Appalachian culture.
He poses that the violent history of those people leads to the culture of the back-country ways. He identifies many cultural memories that seem to come from the Scottish Borders.
Again they're broad statements and simplistic. English culture was so peaceful in the same time period, wasn't it? England, becoming Britain, had a dramatic change in philosophy between 1745 and 1760 that surprised the colonies. However, prior to 1760, there wasn't a significant difference in violence. But I digress...

For the sake of this discussion, Fischer does point out that Scotch-Irish is an Americanism. People back then identified themselves as "Scotch" or linked to England rather than either Scotland or Ireland. The Irish at that time were going through severe discrimination in many places that Scots and, of course, English did not have to put up with. Then there's the other group, the Highlanders, for Scotch-Irish refers more to the Lowlands.
Somehow, Fischer keeps them all as one group. Even though he pulls from a large population base over a long time period, then focuses on a shorter period of time and a narrower settlement area, this is still a valuable reference book. Nobody, up to that time, had undertaken such a sweeping project.