"It's related to The Grey Cock, or The Lover's Ghost."
There is some dispute as to the facts of this. Hugh Shields wrote an article claiming that most versions, including the the Irish ones are more akin to a broadside entitled 'Willie O'. We recorded a beautiful version of it from Wexford Traveller Bill Cassidy called 'Biscayo' which we included on our selection of Traveller songs, From Puck To Appleby; as follows (with note):

15- Biscayo
(Roud 179, Child 248) Bill Cassidy

For he come creeping when I being sleeping,
Down to my old window, was down so low,
Saying, "Who is that at my old bedroom window
That is knocking so boldly and can't get in."
"For I am here, I'm your own true, lover,
I am here this three long hours and can't get in."

Saying, she raised up from her soft down pil¬low,
She've opened th'ould door lads, And she've let him in.
And with love and kisses
How they blessed each other,
Oh, when this long night being slipping in.

Saying, "I must go, I can stay no longer,
For I'm only th'ould ghost of your ould Willie O."
Saying, "What have took your old lovely blush¬es,
Or whatever ate your grand cheeks away?"

"For th'ould cold, cold sea took my lovely blushes,
And it's the worms ate my ould cheeks away."

"I must go, I can stay no longer,
Into a bay called Biscayo.
Where I'll be guided without hand or pilot,
For I'm but the ghost of your Willie O."

"I must cross o'er th'ould burning mountains,
That's in to that bay called Biscayo.
That's where I'll still be guarded*, ah,
Without hand or any pilot,
That's why I'm th'ould ghost
Of your ould Willie O."
[* guided]

We have always thought this song to be a version of The Grey Cock, (Child 248); however, ballad scholar Dr Hugh Shields has cast serious doubt on this assumption. In two detailed articles on the subject, he argues convincingly that it is a version of a nineteenth century Irish broadside entitled Willie O, the main source of which appears to be Sweet William's Ghost (Child 77).
We have also recorded it from another traveller, Katie Dooley; and from West Clare singer Nora Cleary.
Katie Dooley's text, similar to Nora Cleary's, has obviously evolved from the broadside, but Bill Cassidy's text and tune are reminiscent of the well-known version entitled The Grey Cock which was recorded in the early 1950s from Mrs Cecilia Costello, a Birmingham woman of Galway parentage. Whatever the truth of the matter, all three have in common the lover returning from the dead and the couple's time together being brought to a close with the crowing of the cock.
Both Mrs Costello's and Bill's versions have powerful images symbolising the difficulty of the dead returning; in Mrs Costello's, the lover has to cross 'the burning Thames', while in Bill's it is 'the burning mountains'. Unusually, Katie Dooley's version ends with the woman's death and leaves her 'sleeping beneath the billows'. This may be a mistaken substitution of she for he, but it makes perfect sense in the context of the song.
Bill was one of a number of Travellers who liberally scattered the word 'old/ould' into the texts of his songs!

Ref: Dead Lover's Return in Modern English Ballad Tradition, Dr Hugh Shields: Jahrbuch Fur Volkliedforschung, 1976;
Grey Cock: Dawn Song or Revenant, Hugh Shields, Bal¬lad Studies, Folklore Soc. Mistletoe Series, 1976.
Other CDs: Nora Cleary - Topic TSCD 653;
Cecilia Costello - Rounder CD 1776.

My own personal favourite is the Scots Bothy version, 'I'm A Rover' from which the supernatural element has totally disappeared and the dead lover becomes a fairm servant who has to return to his work next morning saying;

"Remember lass, I'm a plooman laddie,
I am a servant and I must obey"

Jim Carroll