Grand Canal Graving DocksView on Google Maps
Used for the repair and construction of boats. Built 1796-1800 and in use until ~ 1960, apparently. They are blocked to the public off, but badly, and you can pass yourself around the fencing at the waterfront when you approach from the South Dock Road.
The biggest dock is filled with water. It holds a derelict ship, the Naom Éanna, home to pigeons. Water in the smaller one closer to the Liffey is much lower.
There’s a good Dublin Inquirer article with a lot more information here.