I know a very leetle bit about language and the way it gets twisted by accent, vocal mannerism (think of Sean Nos with all the grace notes and elongation of words) and then mis-hearing. (Then you have the problem of re-interpretation of apparently "meaningless" phrases by the well-intentioned!)e.g From The White/Green/Blue Cockade - "a shilling from the Crown" has become "10 shillings and a crown", a friend of mine sings "Me Father Gave I Watch and Chain" with the line "I lay beside my working clothes" What?! Of course it should be "I laid aside my working clothes".
"O sleep valiant mountain" could very well be "On the steep slopes of the mountain" or something completely different. My (very long-winded) point is that you are not necessarily looking for geographical features.