There seems to be a common belief found in Youtube comments and elsewhere that the ballad Barbara Allen is Scottish in origin and that the fact that the Scots versions includes 'Martinmas' in the lyrics somehow proves this.

YouTube comments? This initial premise seems to be flawed. I can't find any such lyrics as suggested.

Broadsides are a solid source of these songs (heard a great lecture by Lucy Skeaping on this subject when she visited Texas years ago).

British Library Ballads and Broadsides

Smithsonian Folkways - British Broadside Ballads in Popular Tradition

Bodleian Libraries - Broadside Ballads Online

Harvard Library - English Broadsides: The Bute, Huth, and Percy Collections

These are a few starting points.