Sir Hugh Robertson, 1874-1952. He wrote the Mingulay words, but adapted an older tune, as pointed out by Sarah U. The tune does not seem to have been used for a lyric in any Civil War composition.

"Creag Guanach," the poem by D. McDonald, was posted by George Seto in thread 10414: Creag Guanach
The thread includes posting of "Eemis Stane" by Jim McLean