Which is but one reason many Europeans have an ill concealed disdain for us.

Steve Blair:
The whole "Scots-Irish" thing is, IMO, really overdone. It MIGHT be there if you really want to reach for it, but I honestly don't think that anything Rosen is talking about is especially "unique" to Americans. If anything I might take the position that our reputed bellicosity has its roots in the (possibly perceived) existence of outside threats to the existence of the nation and from emulating the example of the British in many ways.
I'm inclined to disagree on the extent of the Scotch Irish infusion for the reasons I outline below. My personal; view is that there is a streak of American bellicosity and that it is the result of a number of things and the Scotch Irish influence is only one and not that critical. I think you're correct in that much of it hinges on outside threats -- or even the perception of them -- and that is a British inheritance. As are the Scotch Irish...

JMM:
Except for the religious differences, it is hard to see that much difference in how the Scots-Irish and Famine Irish reacted to challenges.
The Scotch Irish and Irish reactions to challenges are similar but there are two major differences. The Scotch Irish don't forget and forgive, the Irish do. One but not the only formative difference was religion, Presbyterianism is not for the faint of heart...

Another was that while the Irish were mistreated by the British, the Scotch Irish were mistreated by the British and the Irish, had been frequently betrayed by both and when they came to America, quickly found that they were despised by the Puritans, the Anglican, and the Catholics (or those Welsh Methodists...) -- so they, used to fighting, moved to the border lands and away from the coasts to get land of their own and if that meant fighting Indians, so be it. Thus New Hampshire, western PA , VA and the Carolinas got settled and these folks continued to move west as the nation looked that way. They kept fighting Indians and other American as well as each other -- but any fight between them was put on hold if anyone even looked as though they might interfere or take advantage of the fight to do something.

The Irish and Germans, as Steve said, joined the Army in large numbers -- the Scotch Irish did not; fighting was fun, not work and people telling you what to do reminded them of the British and those snooty Anglicans -- but oh, by the way, give a War -- they'd appear. The Revolution was fought by large quantities of Scotch Irish, each subsequent war has seen a little less obvious participation as other ethnicities proliferated. But they're still out there and some, like me also have some German (thus it didn't offend me to say zu befehl, Hauptman), some English, Welsh and pure catholic Irish. Since I tend to overreact to minor and even inadvertent slights, condescension, provocation or insults, I would suspect the Scotch Irish quotient to be quite high even if I didn't know it was the predominant blood line on both sides. Point is that the mixing makes us what we are -- but that Scotch Irish distrust of "others" (ANY others...), expectation of perfidy and adherence to Family ('my people') pervades us all. The Scotch Irish in early America were noted for their wanton ways -- loud, rowdy and very tough girls, and the genre itself for the huge numbers of kids they had and their willingness, unlike the Catholic Irish and Lutheran or Catholic Germans (much less those Anglicans) to hop in bed with or marry outside the clan or sept (they used both, septs belonged to Clans. Some of the MacGregors are a sept of MacGregor of MacGregor, others of Clan Campbell).

Oh, and those Europeans -- they also think we're loud, rowdy and excessively tough...