We now have a three-way muddle.

"The Battle of the Somme" is a 9/8 slow march written by William Laurie during WW1. He was in the battle, wrote the tune in hospital after it, and died of his wounds a few months later; he lived long enough to see it adopted by every pipe band in the Army. (And to forfend yet another confusion, he was not the much better-known pipe tune composer Willie Lawrie, who outlived WW1 by a few decades). I've never heard any words for it; there certainly aren't any dating back to WW1. It's played at different speeds; ceilidh bands most often use it as a quick march for the Gay Gordons.

"The Freedom Come-All-Ye" uses (an adaptation of) another WW1 slow pipe march, "The Bloody Fields of Flanders".

"The John Maclean March" is a different song, and it uses a tune which first occurred in print as a fiddle tune, "Brave Scotland", in Keith Norman Macdonald's _Gesto Collection_ of the 1880s.   Some time before WW1 it was in use as a pipe march under the title "Scotland the Brave". Henderson used it for "The John Maclean March" a couple of years *before* the familiar words to "Scotland the Brave" were written, by Cliff Hanley. In the _Rebels Ceilidh Songbook_ he doesn't name the tune, just says "traditional", so maybe he didn't know the name for it then; it may not have been that familiar by name outside the piping world.