Its paper. White onlays are common on the finest Irish bindings. They used to
believe they were vellum, or possibly white leather (whitleather, alum-tawed
skin), but that apparently came from poor observation plus a feeling that paper
was too commonplace. Some writers still say that some of the white onlays might
be vellum or whittawed; it can be hard to tell, especially when (as in this
case) there is more gold than paper showing. But a recent description of this
binding says "paper" quite clearly.

Tom
 
      From: Ed Minch 
 To: Thomas Conroy  
Cc: "oldtools@s..."  
 Sent: Thursday, September 3, 2015 3:58 AM
 Subject: Re: [OldTools] Personal satisfaction
   
Tom
Beautiful work - what is inlaid in the surface, the lighter colored stuff?


Ed Minch

and this
https://www.cs.tcd.
ie/John.Byrne/libproj/book4.jpg