Here's what the late Sean O'Boyle had to say in the sleeve notes to Sarah's monumental Topic album.

Everyone knows the tragic story of young Jemmy Groves and Heard-Hearted Barbara Allen. One look through the list of texts and tunes given in Cecil Sharp's English Folk Songs of the Southern Appalachians will show its widespread popularity. It is recorded in Shropshire Folklore (p 543), Folksongs of the Upper Thames (p 204), Folksongs from Somerset (no 22) and in Gavin Greig's Last Leaves (No. 32), in Mackenzies Ballads and Sea Songs of Nova Scotia (No. 9), in British Ballads from Maine, in Traditional Ballads of Virginia, and in Folksongs of the Kentucky Mountains, and elsewhere. In all, more than 200 variants of the ballad are known from printed sources and and recordings. This version from Keady, Co. Armagh is as good as any I have heard, and it differs from all of them in one remarkable respect. Most versions place the tragedy
"in the merry month of May when the green buds they are swelling",
but Sarah's song has a more sombre and appropriate timing:
"Michael's Mass (Michaelmas) day being in the year,
When the green leaves they are falling,
When young Jemmy Grove from the North Countrie,
Fell in love with Barbara Allen."

The melody is in the Re Mode.