This one is in the DT. I learned it from my father. I don't exactly know if it is Celtic or not, but it mentions Hallowe'en.

FAITHFUL JOHNNY

When will you come again, my faithful Johnny?
When will you come again, my sweet and bonnie?
When the corn is gathered, when the leaves are withered,
I will come again, my sweet and bonnie; I will come again.

Then winter's winds will blow, my faithful Johnny.
Then winter's winds will blow, my sweet and bonnie.
Though the day be dark with drift, that I cannot see the light,
I will come again, my sweet and bonnie; I will come again.

Then will you meet me here, my faithful Johnny?
Then will you meet me here, my sweet and bonnie?
Though the night be Halloween, when the fearful sights are seen,
I will come again, my sweet and bonnie; I will come again.

(Repeat first verse.)

I'd definitely recommend Tamlin, which also specifically mentions the superstition about faeries and what not walking our world on Hallows Eve. Another good Child Ballad for this time of year in Young Benjie (# 86A in the Dover edition) I did a search for this one but couldn't find it. I'll post the lyrics in another message--they're long!

HTML line breaks added. --JoeClone, 24-Jun-02.