I learned this great song at the same time as "The Earl of Murray" and it can also be found in the Clancy Brothers' The Irish Songbook. The songbook advises, " moderately, with great strength" and that is exactly how I sing it dear surfer, the emphasis (mine) being on the great strength.
The song is best summed up by a quotation from Liam Clancy:
"McPherson is one of my heroes. A man who can get an exultant sense of victory and defiance in the face of defeat, and even in death, is indeed a man to be admired."

Lyrics