As Stewie said above, the "Digitrad and Forum Search", always the place to start before asking about such well-known songs as this, reveals many past discussions archived here, besides a set, with tune, in the DT:  Shenandoah

Probably the most comprehensive discussion is the following, to which Jeri provided a link last time someone asked this question (about 3 months ago):

Song info: "Shenandoah"

It contains a great deal of interesting information, and speculation, which there would be little point in repeating here.  Watch out, too, for one particular message containing one of the most extraordinary pieces of pseudo-celtic romantic nonsense that I've seen here in a long time, so completely barmy that it was a pleasure to read.

Malcolm