To be bluntly honest, most whites just don't know where they're from. Or are just such a mutt that it's impossible to tell. My dad's family is supposedly Irish (although we're not sure, there's no family bible and the basis of our ancestry was my great-grandfather who supposedly said once "we're Irish"; most likely I think we're Scotch-Irish) and my mom's family is German.
I've found that looking at last names of ancestors helps. My mom claims she's 100% Irish, but based on last names and recollections of accents (and also the fact that my maternal grandmother was Presbyterian before she converted to Catholicism to marry my maternal grandfather) it would seem that she's at most 25% Irish and at least 75% Scotsmen-who-oppressed-the-Irish-and-then-went-to-America-and-blended-in-with-the-other-Irish-Americans.
