Hi, Robinia - it looks like it's the same as "The Earl of March's Daughter," which is in this thread (click). If that thread is correct, then the song was written by Lionel McClelland of the group Blackeyed Biddy, and it must be fairly recent. There's a recording of the song by Aoife Clancy that appears to be quite popular.

I haven't heard the song, so I can't say more. Note that the Child Ballad "Earl of Mar's Daughter" is listed in the crosslinks above, but that is not related to the song you're seeking. It just has a similar title.

-Joe Offer-