The irony is that poetry, written on paper, is bound to outlast stone and marble monuments. This sonnet is about a young man and there may be an implication that a poem about an ‘everyman’ will outlast a monument to a ruler. War will destroy these monuments, but the irony is that “war’s quick fires” cannot destroy the eternal memory recorded in poetry. Obviously, paper burns more easily than stone. Poetry has an eternal potential because the words on the page express an essential abstract meaning, a poem can be passed on verbally in cultural memory; therefore, poetry is both physical (on paper) and metaphysical and can escape the erosion of time. Shakespeare is also making the point that both the subject and the author of the poem can achieve immortality in this way. However, the secondary irony is that we have no idea who the young man, the subject, of the poem is. But we all know Shakespeare. So, he was half right.
Sorry about that, some freinds got a hold of my account