Are you sure you aren't confusing two songs?

The song LONELY BANNA STRAND (in the DT) has these lines:

Said he, "I'm Roger Casement. I came to my native land.
I meant to free my countrymen on the lonely Banna Strand."

--but it doesn't mention Mandela, Sands, etc.

Then again, maybe your songwriter was quoting this song in his song about Mandela.

Anyway, I can't find any songwriter named Danny Hernan, and I can't find a single web page that mentions all four of the people you named: Mandela, Roger Casement, Bobby Sands, & Padraig Pearse.