I agree the song lyrics that we are familiar with (and were taught as children in school) reflect the sentiments of having Mother Jones coming to visit--but I'm not certain the song is about her. If I am not mistaken, these lyrics predate Mother Jones' organizing days. But I could be wrong about that.

My understanding is the song is one version of the African American spiritual "When the Chariot Comes" (as I stated above. There are other references--like killing the red rooster in the lyrics which obviously have some sort of cultural meaning as well, and likely the six white horses should be looked at in that broader context to that stanza to understand the song's cultural meaning to the community that wrote the lyrics. White horses and red roosters, the number six--all those things would have cultural significance to the folk community the song came from.