That ballad is (probably) the text in Helen Creighton's Songs and Ballads from Nova Scotia:

O bring to me a silken gown,
A web that never went through,
And you must get me a priest unborn
To join us one and all,
Before I lay one night with you
At either stock or wall.

Oh, my mother's got a silken gown,
A web that never went through;
Melchisedec is a priest unborn,
He will join us one and all,
So shake you up that old straw bed,
You must lay next to the wall.
HC comments that "This may be a later refinement of the Caesarean birth of the Child variants." If so, it's rather ingenious, I suppose; but it's still a bit esoteric, to say the least...