Conrad can be careless about crediting his sources. This is copied from one of Gwen Polwarth's books; either Folk Songs of Northumberland (1966 and 1967) or Come You Not From Newcastle (1972). She got it from Thomas Hepple's MS of c.1855 (ref. Roud Folk Song Index, number 2614).

You can see several broadside examples at Bodleian Library Broadside Ballads:

Rest! warrior, rest

Unusually, the Catnach set includes staff notation.

You can see sheet music published in the US at the Lester Levy Collection:

Rest! Warrior Rest; the composer [of the music] is named as M Kelly. Other sources specify Michael Kelly (1762-1826, and credit the words to W. Dimond.