I believe the last verse reference, "And I'll write some words upon the wall so everyone will know" is the author's way to tie Plunkett's own words, to the song. As said in the thread above, it is not uncommon for the "I was here" sort of graffiti to accumulate in prison cells, and as we know, the real Joseph Plunkett penned a poem that in deed left behind his deepest thoughts to share with others ... and that poem expressed the thought that he 'loved so much that he could see His blood upon the rose'

I believe the author pictures Plunkett penning those words on the prison cell walls.