I have no literary support for this, but I believe I can offer insight into what arruginated means. The word "ruga" or its more common plural form "rugae" means in medicine "an anatomical fold or wrinkle especially of the viscera." The lining of the stomach is full of little ridges called rugae.
http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/stomach/anatomy.html The word's relation to the more commonly-encountered "corrugated" should be obvious to this community. I believe that Joyce was describing the appearance of an old-fashioned key, which would have had teeth that were cut in a series of ridges or rugae, with "arruginated" being to rugae as "irradiated" is to radiation.
http://thehouseofabalone.com/skeletonkeys.html http://www.answers.com/topic/key http://www.istockphoto.com/file_closeup/objects/314821_skeleton_key.php?id=314821 
