On the Scots-Irish (or Scotch-Irish) in Stonewall Jackson and the American Civil War by G.F.R. Henderson. This General Jackson and the one who was President were from this fierce race.
The grandmother Ken and I share was a Johnson, the Scots-Irish type and not the Scandinavian, first cousin to the Jacksons and everyone else from the Lowlands who had "son" at the end of their family name. That grandmother married a Stokes (Walter Levi), and his grandfather was Greenberry Jefferson Stokes, and that family has its share of Scots-Irish blood. Grandfather Greenberry was in General John B. Gordan's "Raccoon Roughs" and appears in this 1889 reunion photo. A copy of that photo is among some old papers of my dad (Walter Johnson) that I have. The Wiki bio of General Gordon I linked states that he "was descended from an ancient Scottish lineage . . . "
On Carol's side are the McKinneys, another Scots-Irish family (probably the same family as the McKenzies). I have seen significant evidence over the last 40 years that, in terms of being fierce, the McKinneys easily trump the Johnsons and the Stokeses.