How does one connect to a schema within a PDB (other than schema SYS)?
Turns out there was a typo error in the tnsnames.ora file - the HOST ip address field was invalid for the system in question. 
Example below (values have been anonymized for security reasons)

PDB =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = 10.10.1.11)(PORT = 1555))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = PDB)
    )
  )


The file was probably copied from another server (as a template) and the HOST ip never updated.

Given the above, the correct way to connect would be:

conn myschema/mypasswd@PDB;