I think you're talking about the Rusted Iron Ring?
This ring is pretty useful as it eliminates the movement penalty from moving around in water, or the swamp in Blighttown.
To get it, you must unlock that door using the Undead Asylum F2 West Key. This key is found in the Firelink shrine on a roof near the area you must navigate to return to the Undead Asylum.
This video shows you how to get back to the Asylum. You can also use this written guide on how to return to the area (courtesy of the Dark Souls Wiki):
"To do so, you must have access to the elevator that goes down to Firelink Shrine from the Undead Parish.
Get on the elevator from Firelink Shrine and, as the elevator starts moving up, you must move forward and drop on top of the area where you exit/enter the elevator at the bottom. Follow the path to the right, on top of a little green patch, then look down and you will see a broken column. Roll towards that broken column and land on it, then go across the beam. From there climb up the stairs all the way to the top where you will find the Snuggly the Crow's nest. Step in to it and you will get a prompt to 'Curl Into a Ball'. If you crawl into a ball and wait about 30 seconds, the Crow will pick you up and bring you back to the Undead Asylum."
Since you can freely return to the Asylum whenever you want to through this method, I do not think any item is "missable".