It's a great harmonica tune, but here's the rub. If the chaps are playing it in D, then a G harmonica works best (in other words, you are playing it cross-harp). Likewise, if it's being played in E, an A harp is best. In the latter scenario, all the notes of the tune are present in the diatonic scale of A, which, I'm led to understand, is hunkydory on the fiddle. Talk of the key of E may be better considered in this context...