The music is by Caradog Roberts (1878 - 1935). The words are by Henry Lloyd (1870 - 1946), better known by his bardic name of "Ap Hefin".
"Rachie" (or "Rachel") was the daughter of Caradog Roberts. "Ap Hefin's" words are a call to battle.
Caradog is yet another famous resident of Rhosllannerchrugog, a small town that was one of the centres of the Welsh religious revival of 1904-1905.