The endpaper of Wayland F. Dunaway's book The Scotch-Irish of Colonial Pennsylvania (1944) might to some today appear to be a cliché, but as recently as 2010 it has also been described as one of 'the best books' on the subject.
He was born in the town of Kilmarnock near the coast of Virginia in 1875. As well as an academic he was a Baptist pastor, and wrote an article defending what he called 'Scriptural Baptism or the Immersion of Believers'.
•The Scotch-Irish of Colonial Pennsylvania is a great book and is available online here
• I presume that this book is by Dunaway's father, of the same name, a Confederate army veteran.
• An unpublished 2009 PhD thesis by Peter E. Gilmore, superbly entitled Rebels and Revivals: Ulster Immigrants, Western Pennsylvania Presbyterianism and the Formation of Scotch-Irish Identity, 1780–1830 is a source I come across fairly often and is online here.
• Gilmore's paper From Rostrevor to Raphoe: An Overview of Ulster Place-Names in Pennsylvania, 1700-1820, also looks brilliant.
• His article on the music of Western Pennsylvania is packed with information.