John, here's what I found on the song, the event and John MacLean's story:

[1977:] The John MacLean March had its first performance in the St. Andrews Hall, Glasgow, at the memorial [concert?] to the great Scottish republican socialist on 28 Nov. 1948, where it was sung by William Noble. It is set to a traditional version of a pipe melody that is today played and sung all over Scotland to the more commercially patriotic verses of Scotland the Brave. The song is notable for its gradual build-up to the conclusion, where another of Henderson's mythic figures - the proletarian teacher and leader, imprisoned and tortured for his beliefs - rests and sleeps in an ordinary worker's home. The singer is a representative Clydesider addressing firstly other lowlanders like himself ('Mac', 'Jock' and 'Jimmy'), next a Highlander and an immigrant from Ireland, in a wonderful rendering of facetious working-class camaraderie which enables the Hero to be seen as in principle no different from those who are welcoming him - from 'Wull' who 'grips his banner weel (that boy isna blate)', and which therefore makes the other heroes who are Maclean's mates, Lenin and Liebknecht, our mates too. In the second last stanza 'Glasgie, oor city' becomes 'the haill world beside'; next, at the beginning of the last verse, the vision contracts to a little room and a domestic scene (once more, the values of Burns's Cotter!) where the hero rests with his freens, before expanding into the resounding crescendo of the monosyllabic hammer-blows to which all Scotland will march now 'Great John Maclean has come hame to the Clyde'. The final synthesis is between him and us, between Hero and home - an ordinary, small family home where comradeship and sharing prevail. (Thomas Crawford, notes 'Freedom come all ye - Songs and Poems of Hamish Henderson')

[1988:] The great protagonist of adult education was teacher John MacLean (1879-1923), who began his political life in the SDF [Social Democratic Federation] and who until his death conducted classes in economics all over Scotland. MacLean, the son of a Pollokshaws potter, was one of the theorists of socialism as well as being a great propagandist. He wanted to fight the war against capitalism, and not the capitalists' war of 1914-18. In 1918 Lenin appointed him as Scottish Consul to the Bolshevik government. MacLean's recognition of the potential for revolution in Scotland during the Great War and after made him a danger to the government. He was repeatedly jailed for his political opinions and so badly treated that he died prematurely at the age of 44. As well as having a portrait and photographs of MacLean, the People's Palace [in Glasgow] has his desk and some of his personal items, including his university passes and literature from the Scottish Workers' Republican Party which was founded by him. (Elspeth King, The People's Palace 72)

[1988:] He was a schoolteacher [...] too old himself to go to the war, but he advised the young men of Scotland not to go fighting but stay at home and help the country from the inside. He was taken to court, found guilty of sedition and sentenced to seven years in a very tough prison in Scotland. It's hard to believe now but the Glasgow people complained so loudly at this savage prison sentence that the government was embarrassed, and after a few weeks they allowed the man out of prison - very quietly, but somehow the word came back to Glasgow. And when he arrived there were two hundred thousand people to meet him in the railway station. (Intro Iain MacKintosh)

[1990:] Teuchter: mildly derogatory Lowlanders' word for a Highlander. Its etymology is totally obscure and I have never heard a satisfactory explanation. (Sean Damer, Glasgow: going for a song 109)
The Socialist movement in Glasgow was closely tied in with Highland and Irish societies [and] it is not by accident that John MacLean's parents were both Highlanders and victims of the Clearances, his father from Mull and his mother from Corpach; MacLean was brought up on stories of the bitter injustice of the Highland Clearances. (Damer, Glasgow 120)
[There was] the genius and courage of John MacLean who [in Glasgow during the First World War] was everywhere, agitating, organising, educating. At times he seemed like a one-man revolutionary party. Historians like Iain McLean have found it convenient to dismiss him, but Glaswegians are not easily fooled and they turned out in their tens of thousands when he was released from jail in 1917. Hamish Henderson's fine song expresses local feelings perfectly. (Damer, Glasgow 130f)

Standard works on John MacLean by John Broom and Nan Milton, both 1973.