Clive D. W. Feather wrote to uk.transport.london on Mon, 3 Jan 2005:

In article , Mrs
Redboots writes
My husband (who comes from Northern Ireland) can still tell you where a
car with a NI registration comes from, and even I know a few of them: IW
is/was County L'derry, OI was Belfast (city), IJ was County Tyrone, I
think..... Anything with an I or a Z in it was either Northern Ireland
or the Republic.

Wasn't it I for NI, Z for the Republic, S for Scotland, and W for Wales?
Though I don't recall who got hybrids like SI or IZ.

Not entirely, as I know Co. Derry had/has at least one code with a Z in
it, but I can't remember what it was, and Husband is now back at work.
S was mostly in Scotland, I do know. SI was, I think, somewhere in the
Republic and IZ was - sheesh, I'm thinking Derry City, BICBW!
--
"Mrs Redboots"
http://www.amsmyth.demon.co.uk/
Website updated 2 January 2005