Here's some info from Traditional Ballad Index:

Lass From the Low Country/Lovers' Farewell (I)
DESCRIPTION: The girl laments that her love came and bade her farewell, then went to war in the Low Country. He fought, and none knew where he fell. Now "he may sleep in an open grave, But I will wake on my pallet of grief...."
AUTHOR: unknown ("collected" by John Jacob Niles)
EARLIEST DATE: 1961
KEYWORDS: parting death separation grief war
FOUND IN: US(SE?)
REFERENCES (1 citation):
Niles 17A, "Lover's Farewell" (1 text, 1 tune, dubiously labelled as Child 26)
CROSS-REFERENCES:
cf. "The Three Ravens [Child 26]" (lyrics)
cf. "The Highland Widow's Lament" (plot)
Notes: Niles lists this piece as a form of "The Three Ravens," on the basis of a few lyric similarities ("evensong"; "No man knows that he lies there / But his horse and his hound and his lady Mary"; "Oh, he may sleep in an open grave / Where raven fly and flutter"). The plot, however, is completely different, and reminds me more of "The Highland Widow's Lament," which tells of a soldier dying in the Low Country (on behalf of Bonnie Prince Charlie). The piece is quite beautiful, but one can only suspect John Jacob Niles's hand in it. - RBW

-Richie