Blackwaterside - Three songs are possible:
The Irish Girl (Down by Blackwaterside) The Blackwaterside (Sam Henry's Songs of the People p 461) (Laws O1) Blackwaterside (Voice Squad - Many's the Foolish Youth)
This is a surprising list to have been generated by anybody's grandmother. Several (at least three) songs written within the last fifteen years and a lot more which have become known only relatively recently. For example "Welcome poor Paddy Home" went from Willie McElroy to Cathal McConnell to everybody else; Long Shanakyle went from Straighty Flanagan to lots of people but has only become widely circulated in the last three or four years courtesy of Dervish and Deanta.
I'm a natural born cynic; far too many people claim things about these songs which can only be recognised as probably false by someone whose nose has been sensitised by years of poking it into song books and archives.