Thank you all for your quick and intelligent responses! I may have to actually sign up here instead of guest-lurking... :)

Nick is brilliant - and absolutely right, coz I found an old tape the other day and recognized the piece after the first three notes Joan Baez sings.

I found the exact same site linking the tune of 'River in the Pines' to 'Mary's Lament' but then couldn't track its pedigree any farther, as all searches for the lament wound up dead-ending at contemporary compositions about (the Virgin) Mary's grief for her son. Which, of course, the old Irish song may have been as well...

'Lanquesnet's Lament', BTW, before anyone goes off on that tangent, is the title of the newly-composed poem/lyrics which my German friend wrote to go with the tune (now ID'd, thank you!) that was in her head. The lyrics exist only inside our fantasy world of Caelereth (can I put in a plug for www.santharia.com
here?) as do any references you might find in the lyrics, of course!   

Again, I appreciate everyone's input. Feel free to visit us if you like contemporary compositions, because we have a couple of our own composers. I shall certainly be back here - what a wonderful community!

Regards from the bard,
Judith