The poem
The star-filled seas are smooth tonight From France to England strown; Black towers above Portland light The felon-quarried stone.
On yonder island; not to rise, Never to stir forth free, Far from his folk a dead lad lies That once was friends with me.
Lie you easy, dream you light, And sleep you fast for aye; And luckier may you find the night Than you ever found the day.

i think this peom is lame and it sucks for teachers to teach it to their students its nosense and has nothing to do with poetic justice
this poem is totally talking about justice. an innocent guy is taken to prison (for supposedly no reason at all.. injustice *hint*hint*) on the isle of portland and he is forced, like the other criminals, to quarry the stones. this guy dies from depression, loneliness, isolation and confinement from the outer world, and from working too hard. and ae housman (because he is always depressed) is telling him that he's better off dead and peaceful, than living in this crappy and unjust lifestyle of his.