My understanding is similar to David's above, in that after the English took over what we now know as Northern Ireland, those granted land were required to populate them with non-Irish citizens of the then UK, generally lowland scots, displaced highland scots, and northern english, the most readily available and interested potential immigrants, with the intention of driving out the native Irish population from these lands. They were generally grant land leases of thirty years, land not available to them in Scotland, and they flourished in their new environs for generally one plus generations. Problems came when those leases lapsed, and many landowners, seeing more profits from newer opportunities for use of their land, displaced these "ulster-scots" yet again, some who stayed on in other capacities, and others who then left for America well before the major "irish-irish" immigration waves, to become subsequently known as "scots-Irish" in later America. Not being entirely welcomed by the newly landed English gentry along the coasts in the mid-Atlantic colonies they were welcomed to settle the "frontiers" of the western mountains, the Appalachians and Alleghenies, where they finally gained landownership generally for the first time in their family memories. That landownership came at the price of acting as a buffer between the rapidly becoming intolerant and belligerent native american tribes and the coastal landed English gentry, but brought with it the isolationism(from historic English strict rule) and independence of lifestyle and religion long sought after by these now habitually displaced people. Settling the mountains from southern Pennsylvania through western Maryland, central and western Virginia (now West Virginia), western North and South Carolina, well down into northern Georgia, and then subsequently western into what became West Virginia, Kentucky, and Tennessee. Although by no means exclusively, most of what subsequently became the "hill-billy" populace and culture derives from this scots-irish seeding, mixed with later immigrant waves. Although these peoples were only a couple generations from their Scottish heritage, and with no real long term adoption of Irish heritage, and generally "undesirable" in proper coastal English society of the pre-Revolutionary, they were a people with profound newly discovered independence and absence of immediate allegiances except to one another, hence the isolated hillbilly culture that has developed since both in reality and in popular cultural impressions. The use of the term scots-irish is really one that has become popularized in the 20th century and since as a way of describing these original immigrants, and subsequent waves of holdover ulster-scots that arrived over more than a hundred years before, during, and after the US Revolutionary War, and to separate them from the later massive and temporally compacted immigrations of Irish-Irish in the early to mid 1800s. Many historians credit the scots-irish and their offspring as accounting for as much as 75% of the enlisted ranks in the US revolutionary army (compared to 80-90% English or Scottish in the officer corps). Their descendants also likely made up a large majority of enlisted ranks of the confederacy during the later American Civil War (War Between the States, or War of Northern Aggression to many old school southerners) while the union army had a far greater melange of english descendants with all the other ethnic immigrations endemic to those states (Irish-Irish, Germans, swedes and Norwegians, etc...). Statistically the scots and scots Irish make up probably the largest cultural immigrant group ever in America, aside from the original founding English, although not as easily or often recognized as their immigrations occurred over 150 years in multiple early and small disorganized waves, compared to the more recent temporally compacted ethnic immigrations of the other European and Asian cultures that came later, even to current day.