The version on the DT is from Jean Redpath's recording; she doesn't name a source, other than to say that it's Irish.  Probably most people who sing it nowadays are using the version recorded in the 1960s and '70s by, most notably, Sandy Denny and before her, Bert Jansch.  Jansch got it from Anne Briggs, who in turn -so far as I know- had it from A.L.Lloyd.  Lloyd may have got it from the BBC Sound Archives' recording (made by Peter Kennedy and S. O'Boyle in 1952) of Paddy and Mary Doran.

Peter Kennedy gives a version, Down By Blackwaterside, in Folksongs of Britain and Ireland.  That one came from the traveller Winnie Ryan, (Belfast, 1952), and has pretty much the tune we all know.  Versions with much the same text (but different tunes) were collected in the West Country around the turn of the century by, among others, Baring Gould (The Squire And The Fair Maid) and Gardiner (Abroad As I Was Walking).  The issue is muddied by the fact that there are other, overlapping songs such as Captain Thunderbolt (Down By The Shannon Side) and Down By The Riverside and another song called Down By Blackwaterside (The Irish Maid) which has a quite different story.  19th century broadsides of most of those can be found at the Bodleian Library site.  Kennedy is inclined to think that the English betrayal song found its way to Ireland, where it picked up the Blackwaterside locale from that song, and an Irish tune from...well, somewhere or other.  So far as I can tell, this hasn't been discussed in the Forum before.  Anybody else?

Malcolm