Library of Congress does not really try to make a distinction between "folk" and other music for classification. It it very geographically driven.
If you run down to the Library of Congress and browse at M1627 you'll find collections of secular vocal music not limited to one particular place. You'll find things like "Folk songs of the world" but also "National anthems, psalms and hymns."
You will not find anything from one-three specific geographical locations. US songs will be one place (mostly M1629 but also M1630.18), Irish American (M1668.2), Ireland M1744, and so forth. Under each country there is provision for both General, where traditional music usually ends up, and Popular Music. Topical songs are in M1978 so we have Cowboy songs in M1978.C6; Fathers M1978.F26; Sailors M1978.S2.
Fiddle tunes end up with the rest of the solo violin music in M40
There's not one way to classify music that will suit everyone.