As far as I know Tam Lin is essentially a Scottish ballad. The versions in Child, and the oral versions collected from Travellers in the 20th century, are from Scottish sources (I would be fascinated to hear that there were ever any traditional English/Irish/Welsh variants - but I doubt it).

Back in the 1960s Bert Lloyd collated a version from various Scottish sources and produced an Anglicised text. I heard him sing this Anglicised version, at Peterborough Folk Club, in about 1968. It proved to be a revelation to a young lad (as I was then) just getting into folk song.