Rick Lee was kind enough to respond to an e-mail query concerning his version of Tam Lin, and told me that he used a modified version of an Irish melody from a song called "The Whin Blossom". He also sent me a lengthy quotation from his liner notes to "Natick" (which, of course, I neglected to read before e-mailing him...) pointing out that Tam Lin "is the resolution of the story of Thomas Rymer and The Wee Wee Man" (both recorded in Child) and is "excellent instruction in how to rescue someone from enchantment". Could these be the origin for the title of the Incredible String Band's "Wee Tam" - I think they are.
Excellent and very detailed instruction indeed. Lee points out that "the earliest printed fragment appears in Herd's Ancient and Modern Scots Songs, 1769, but the story is certainly much older." I have some investigating to do here.