What is the irony in "Nor Mars his sword, nor war's quick fire shall burn / The living record of your memory." Sonnet 55 by William Shakespeare?

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.