The Scotch-Irish were earlier settlers than the Germans in Westmoreland County. The Scotch-Irish spread to the west and into Maryland and Virginia, while the Germans settled mostly in eastern Pennsylvania. Pennsylvania was the center of Scotch-Irish immigration up to the Revolution. The Scotch-Irish took their name from being the descendants of colonists who moved from Scotland to Ireland. They were not really a mixture of the two cultures, but were Scotchmen who for five generations had been away from Scotland. They lived in Ireland because they were originally Scots whom James I settled in northern Ireland on land taken from the Irish and because they fled from the persecution of Presbyterians in Scotland by Charles II and James II from 1660 to 1688. Although they were not an ethnic mixture, they displayed the distinct cultural characteristics of Scotch shrewdness and strict morality and the Irish love of liberty and ready wit.
The Scotch-Irish lost their sense of nationality because they did not belong to either group. This fact helped to make them independent. The Scotch-Irish were disliked by the Puritans, Quakers, Virginians, and the Pennsylvania Dutch because they were more aggressive settlers than these groups. The Scotch-Irish were also very active politically. They were the [SIC] among the first groups to resist royal authority in Virginia and they were among the first to protest against English tyranny in the 1760's. The Whiskey Rebellion of 1794 was also attributed to the "Irish" around Pittsburgh.
Some of the character of the Scotch-Irish is captured by David McCullough in his book 1776 as Washington's troops converge at Boston to meet the British:
One Virginia company, led by Captain Daniel Morgan, had marched on a "bee-line" for Boston, covering six hundred miles in three weeks, or an average of thirty miles a day in the heat of summer.
Mostly backwoodsmen of Scotch-Irish descent, they wore long, fringed hunting shirts, "rifle shirts" of homespun linen, in colors ranging from undyed tan and gray to shades of brown and even black, these tied at the waist with belts carrying tomahawks. At a review they demonstrated how, with their long-barreled rifles, a frontier weapon made in Pennsylvania and largely unknown in New England, they could hit a mark seven inches in diameter at a distance of 250 yards, while the ordinary musket was accurate at only 100 yards or so. It was "rifling" -- spiraled grooves inside the long barrel -- that increased the accuracy, and the new men began firing at British sentries with deadly effect, until the British caught on and kept their heads down or stayed out of range.
Welcome as they were at first, the riflemen soon proved even more indifferent to discipline than the New Englanders, and obstreperous to the point that Washington began to wish they had never come.
One of the major characters featured by McCullough was Henry Knox who played a pivotal role conveying artillery to Boston from Ft. Ticonderoga:
Colonel Henry Knox was hard not to notice. Six feet tall, he bulked large, weighing perhaps 250 pounds. He had a booming voice. He was grergarious, jovial, quick of mind, highly energetic -- "very fat, but very active" -- and all of twenty-five.
"Town-born" in Boston, in a narrow house on Sea Street facing the harbor, he was seventh of the ten sons of Mary Campbell and William Knox, Scotch-Irish Presbyterians. When his father, a shipmaster, disappeared in the West Indies, nine-year-old Henry went to work to help support his mother, and was thus, like Nathanael Greene, almost entirely self-educated. He became a bookseller, eventually opening his own London Book Store on Cornhill Street, offering a "large and very elegant assortment" of the latest books and magazines from London. In the notices he placed in the Boston Gazette, the name Henry Knox always appeared in larger type than the name of the store.
 Henderson RC. "The Star of the West:" Westmoreland County, Pennsylvania, From the Colonial Era to the Present. Westmoreland History. 2(2):44, 1996 Summer.
 1776, David McCullough (New York: Simon & Shuster, 2005), p. 38.
 Ibid., 58.
For more background about the Scotch-Irish including the derivation of the name itself, check out these sites:
Our Scotch-Irish Heritage
Additional note - as the nations of Scotland and northern Ireland came together with England and Wales; so too, did their respective flags become components of the flag of the United Kingdom.