Old Ironsides was the nickname of a famous United States battleship, the USS Constitution, that fought in the War of 1812. Holmes wrote this poem to protest the news that the ship would be decommissioned. To be decommissioned means to be retired from active duty. The poem helped ensure the ship would stay commissioned, which it still is.
In stanza two, Holmes mourns the fact that the ship, which has seen heroic and victorious battle, will never again go to war. He describes those who want to retire it as harpies. A harpy was a mythological beast with the head of a woman and the body of a bird. Harpies were filthy, violent tormentors.
In stanza three, Holmes states his feeling that it would be better for the ship to be sunk in a storm at sea than decommissioned. He thinks sinking Old Ironsides amid wind and lightning would be a more noble and fitting end than retiring the warship. He writes,
And give her to the god of storms,
The lightning and the gale!
This is an emotional poem that uses pathos or emotion to persuade patriotic readers to feel outraged at the ship being decommissioned.