The poem 'Old Ironsides' refers to the United States Ship Constitution, a battleship that had played a major role in naval battles during the War of 1812. In 1830, when Holmes wrote the poem, the Navy was considering decommissioning and dismantling the Constitution. Holmes was incensed by this possibility and wrote 'Old Ironsides' to express his outrage.
In the second stanza, he recalls the battles she has seen and the warriors who have fought and died on her decks. Using "the eagle of the sea" as a symbol for both the Constitution itself and the United States, he warns that they are in danger of being "plucked" - taken apart, made useless - by the "harpies of the shore" - evil mythological creatures symbolizing those threatening the destruction of the ship.
The third stanza suggests that, if the Constitution must be decommissioned, the rightful end for the noble should be burial at sea. Since "her thunders shook the mighty deep" when her cannons fired in battle, Holmes finds it appropriate that "her shattered hulk should sink beneath the wave" rather than being disassembled on land.