In message , David
Cantrell writes

So why County Durham instead of Durhamshire?

Possibly because the county had several subdivisions that were in
themselves already "shires" (Islandshire, Bedlingtonshire and
Norhamshire were all exclaves that eventually became part of
Northumberland).

There's a sort of general principle that an area which took its name
from an ancient kingdom or tribe (such as Cornwall, Kent, Sussex,
Middlesex, Essex, Suffolk and Norfolk) didn't get a "-shire" appendage,
while areas based on administrative centres did (such as York/Yorkshire,
Hereford/Herefordshire, Oxford/Oxfordshire and so on). But there are
many exceptions (including Durham and, especially, Berkshire).
--
Paul Terry