Soodlum's 100 Irish Ballads Volume 1
explains that Spancil Hill is the place where a big horse race takes place every year on June 24th in Ireland. The song is of a homesick Irish emigrant in California who whilst dreaming returns to his home and the arms of his loved one. He is rudely awakened and brutally returned to reality by the crowing of a cock.
For me, the song is a little on the doleful side and I don't sing it too often.