In the case of traditional folk music from Appalachia and in some instances African-American early folk music, the nonsense words may have emanated from a bastardized gaelic which found it's way into American songs. The Minstrel show employs these silly words which have lost their original meaning in gaelic.

"Bobba-link-a-die-do, king kong kitchie kitchie ki-me-o" et. al may be the earlier gaelic
equivalent of "Mairsy Doats".

Ssgr