Scots-Irish is almost synonymous with hillbilly in the US. Scots migrated first from Scotland to Ulster (not Northern Ireland), then to the US, often entering through Philadelphia, walking west until they hit properly tilted country, then working their way down the Appalachian Mountains (at the time America’s “backcountry”). But many Scots immigrants never detoured through Ulster. More importantly, as David Hackett Fischer points out in Albion’s Seed, a large portion of that immigration wave were from northern England. They weren’t from Scotland at all.
Fischer titled the relevant section of Albion’s Seed “Borderlands to the Backcountry.” As Fischer notes, they may have been mixed Scots and English, but they came from a relatively small geographic area centered on the Scottish-English border, and they shared a common culture. I was convinced by Fischer’s arguments when I first read Albion’s Seed over ten years ago. New knowledge as to my own heritage supports his findings.
My patriline (my father’s father’s fathers’ . . . ) isn’t hillbilly at all, although we have certainly been in the mountains long enough to assimilate. That branch of the family came to the Americas via Jamestown, very early in the migration of Cavaliers described by Fischer. They may not have been Cavaliers—numerically there were far more indentured servants—but they were large enough landowners by the Revolution to pick the wrong side. I say presumably through Jamestown because there were no immigration records at that time, as there were later for, say, Ellis Island. All I know is that one generation was born in the vicinity of London and died in Jamestown.
My paternal grandmother’s patriline, on the other hand stretches back to Scotland. Her people did the usual route from Scotland to Philadelphia (again, quite early in that wave). I haven’t traced most lines back, but I assume they tend toward the hillbilly over the Cavalier.
I did a genetic test through 23andMe a few years ago. The results were unsurprising. Very, very European, mostly from Great Britain. I happened to log back into my account this past weekend and discovered my results had been updated with more granular information. Sure enough, the map shows the concentration in London you would expect based on my patriline, and Great Britain as a whole is represented, with another concentration in Glasgow. The only blue in Ireland is in old Ulster, suggesting I have no Irish heritage, only some Scots-Irish. I say some because the most interesting new information (to me, at least) is that the only concentration outside of the major cities is in northern England, not southern Scotland. Like many of the hillbillies Fischer describes, my heritage is primarily English. Not that it makes much of a difference.
 I largely ignore my mom’s heritage both because she was adopted and because I was raised firmly within my dad’s family’s folkways. Interestingly, though, updated genetic information strongly suggests one of my mom’s natural grandparents was Sicilian.