It seems fairly close to the first half of Scotland the Brave, and I wouldn't be surprised if it was derived. The tune itself may not be named John MacLean's March, because I'm pretty sure I've heard an instrumental tune by that name.

MacLean seems to have been the British equivalent of Eugene Debs (or vice versa!) - both were socialists and anti-war activists imprisoned during WWI. I've never heard any songs about Debs though. I first heard of MacLean in the McAuslan stories by George MacDonald Fraser - a minor character is a die-hard Communist (in 1946) and a platoon sergeant in the Gordon Highlanders. He refers to MacLean's rallies and has a friendly enmity with his son-of-a-baronet platoon commander.

Anyone know the chords, or if the song is under copyright? I wouldn't mind recording it.