"Webb notes that some of the Scots-Irish made their way to Massachusetts in the early 1700s, thinking the Puritans would welcome them as fellow Calvinists."
In 1718 to be exact, five ships arrived from Londonderry, at Boston. Among them my ancestors.
One community they formed is Londonderry, NH. Later mine went to Falmouth, Maine.
Later still (1750s) mine went across to Annapolis Royal, Nova Scotia to help the crown "plant" the north, to diminish the French (Acadian) influence.
In 1856 my ancestors moved from New Brunswick to Monticello, Minnesota. A much different route from the mainstream Scots-Irish (or Ulster Scots as they are called everywhere but the US).
As to mixed heretige, my ancestors in Nova Scotia wrote of their Viking roots. They carried an ancient Celtic name, derived from a village in Ayreshire, Scotland; a sept of clan Douglas.
But these people called themselves "Irish" up in Canada.
The term Scots-Irish only came into use when the famine Catholics came across. These folks had been dubbed the "niggers of Europe."
So the protestants "Irish" that had come over 100 years before coined the term "Scots-Irish" to define their difference from the recent mainly catholic Irish arrivals.
Back in Ulster in 1798, the protestants and catholics rose up together, against the British but were defeated. The northern counties of Ireland have fascinating history, dating to the earliest Christian centuries.
In America many people that came directly from northern England and Scotland wound up falling in among the Scots-Irish and being so labeled.
The Border history is fairly distinct and separate from the migration from Scotland to Ireland.
The Celtic language has two main groups. One is Irish/Scottish. The other is Welsh/Breton. And they play pipes in the Spanish province of Galicia.
My cousin traced our ancestry to washing up on the shores of northern Ireland after the disaster of the Spanish Armada. I guess some finally got back to Spain.