Lyrics (the verse added by Mudguard is not in the original poem) and a nice midi are at: Lark in the Clear Air
Barry Taylor also has a nice midi.
Sir Samuel Ferguson was the composer of this popular song in its usual form, but often recordings and website comments list it as traditional. Supposedly a translation from the Irish, but I read somewhere that Ferguson had a bit of John Jacob Niles in him. There are several musical settings for it.

The Cambridge Singers, conducted by John Rutter, have a grand version (on cd) which they refer to as traditional.