In sailing jargon, the phrase “in irons” means the bow of the boat is pointed into the wind and is unable to move- in a sense, the boat is fettered and shackled. The boat may drift with the tides, but no wind will push it.
Magwitch seems to have a habit of landing himself in both boats and irons, and boats in irons. When we first encounter him, he has escaped a prison ship and is fleeing across the moors, hauling his chains with him. Prison ships at the time were often hulked ships; vessels that were decommissioned from formal use, but still in decent enough shape they would be moored offshore and used as storage, sometimes for goods, and sometimes, as in the case of Great Expectations, people. Relatedly, as we briefly discussed in class, these specific prison ships were likely once slave ships.
To turn a ship out of irons, the crew must turn the sail so it catches the wind again. A person in irons must wait until someone else frees them- Magwitch relies on Pip to bring him a file from the forge. He ends up being caught soon after, and we later learn Magwitch has been carted off to Australia. Later, during the plot to escape to Hamburg, Magwitch ends up on another boat:
“If all goes well,” said I, “you will be perfectly free and safe again, within a few hours.”
“Well,” he returned, drawing a long breath, ” I hope so.”
“And think so?”
He dipped his hand in the water over the boat’s gunwale, and said, smiling with that softened air upon him which was not new to me: “Ay, I s’pose I think so, dear boy.”
The irony of it all is that despite literally and figuratively being in irons, Magwitch has made progress- or so he thinks, because for all his efforts, he never truly escapes for long. We know how the plot ends, with Compeyson dead and Magwitch once again caught, and once he passes away in prison, even the fortune he saved for Pip vanishes. A Sisyphean tragedy with a gothic façade- even a ship that has been hulked will one day end up in the scrapyard.