The Collaborative International Dictionary of English v.0.48:

Glyn \Glyn\, Glynne \Glynne\, n.
   A glen. See Glen.

   Note: [Obs. singly, but occurring often in locative names in
         Ireland, as Glen does in Scotland.]
         [1913 Webster]

               He could not beat out the Irish, yet he did shut
               them up within those narrow corners and glyns
               under the mountain's foot.         --Spenser.
         [1913 Webster]