i always thought it was a song about the english slaughter of the irish.
in an irish county which turned the valleys red with irish blood

something like the blood flowed like a river through the red valley i cant remember the exact words but its from the 1700s and was taken to america in about 1810 a irish rebel song, and has changed and changed over the years.
thats what my granpas granpa told him while laughing histericly at gene autrey singing it in the cinema about 1930. he said its an irish rebel song about the slaughter of irish rebels by goverment troups in 17 somthing.
my granpa told me this about 30 yrs ago so its a bit vague