A majority of Irish people who emigrated to America in the 18th
	century were Protestants from Ulster.  Most of these, in turn,
	were descendants of settlers brought in from Scotland from
	the 17th century during the so called plantation of Ulster.
	(Being Protestant, it was believed they would prove more loyal
	than the troublesome Irish.)  "Scotch-Irish" usually refers to
	those emigrants or to their descendants.  (Note that most Scots
	do not like being called "Scotch" nowadays, because this word
	is usually used for whisky from Scotland.)