The song isn't yet entered in My Songbook, which Brucie tried to link to. Here are the CD liner notes:

[1996:] The Ludlow Massacre is one of those shameful occurrences in social history that make scarce an imprint on your average history book. While history books are busy talking about this battle or that war, they usually pass no comment on the social uprisings that took place here, there and everywhere in the first 20 years of this century.
Briefly, the United Mine Workers of America had managed to organise the many different races of newly-arrived immigrants in the coalfields of Southern Colorado by 1913 to the extent that it was decided to strike against the mighty Colorado Fuel and Iron Company. This company was controlled by John D. Rockefeller and the conditions the workers were kept in resembled the unspeakable squalor of Medieval feudalism.
The company, of course, owned the ramshackle huts that the miners and their families were packed into and evicted them at the outset of the strike. The Union moved them into tent colonies, the largest one being at Ludlow.
In the icy winter of 1913-14, the striking miners and their families were controlled in their tent town by the National Guard which had been called out as a buffer between the Company Guards and the miners, but which soon sided with the company. The dominant figure in this State Militia company was Lieutenant Lindersfelt, an especially brutal bully boy. On April 20 1914, with machine guns and high-powered rifles, the militiamen, who may have been drinking, opened fire without warning on the Ludlow tent colony. The miners had dug pits under some of the tents, so that pregnant women and nursing mothers and their children could be below the firing line. However, the murderous militiamen went berserk and set fire to the tents with coal oil. The next day, when the smoke had cleared, the blackened bodies of two young mothers and eleven children between three months and nine years were found.
Though Lindersfelt and others were found guilty after a farcical trial they were punished only by their promotions being delayed. At a congressional hearing, Rockefeller insisted that he had known nothing of the conditions in his mines nor of the company's crimes committed during the strike. This was later shown to be totally untrue.
At the spot where these 13 people died, today there is a monument - put up by the United Mine Workers, note, not by the State of Colorado - which says "In memory of the men, women and children who lost their lives in Freedom's cause at Ludlow, Colorado, April 20, 1914". (For the whole story see: "Buried Unsung" by Zeese Papanikolas.) (Notes Andy Irvine, 'Rain on the Roof')

I'll try and copy out the whole lyrics some time this week.

McGrath, thanks for the Aberfan link! I'll add it to Palaces of Gold.