The article indicates they moved on to America.
"The principal movement of people from the Lowlands of Scotland to the Northern Ireland Province of Ulster did not occur until the seventeenth century. This migration, estimated to include well over 100,000 Scottish Protestants, mainly took place during the 90-year period from 1607-1697."
"By about the year 1717, conditions had reached such a point that many Ulster Protestants began a new migration; this time to the American Colonies. It is estimated that more than a quarter of a million Scotch-Irish emigrated to America between the years 1717 and 1775!"
Now how did they go from 100k to 250k in such short a time? That wasn't the 20th century when populations exploded based on advances in medicine, in those days, populations stayed the same size.