Here is a message I sent to the webmaster of the London Bus Page ages ago (early 1998) explaining this. It's probably a bit out of date but I would welcome comments and corrections.Just a note to clarify how Northern Ireland Registrations are issued.
The complete list for all the counties is:
Antrim: IA, DZ, KZ, RZ
Armagh: IB, LZ
Down: IJ, BZ, JZ, SZ
Derry City: UI
Derry County: IW, NZ, YZ
Fermanagh: IL
Tyrone: JI, HZ, VZ
Belfast: OI, XI, AZ, CZ, EZ, FZ, GZ, MZ, OZ, PZ, TZ, UZ, WZ
All the above letters have been used at least twice, first as two-letter prefixes and
then as two-letter suffixes. The two letter system ran out in most counties round about
1970, I think. We are now working through the system of prefixing each two-letter mark
with the letters A-Y (except Q).
Antrim is very near the end of the DZ range and might even have started AKZ by now;
Armagh has just started the LZ range; Down is nearing the end of the BZ; Fermanagh is at
OIL, Tyrone nearing the end of the JI range and Belfast is rapidly approaching the
end of the AZ range.
I'm not too sure about the difference between Derry City and County, but UI and IW numbers
are definitely being issued concurrently, so they are different.
I've explained how the system works on my WWW page
(see http://www.geocities.com/MotorCity/Speedway/7884/fleetlis.htm#regno ),
but in case it isn't clear I've shown how the Co. Down numbers have been issued for an
example:
IJ 1 - IJ 9999
BZ 1 - BZ 9999
JZ 1 - JZ 9999
SZ 1 - SZ 9999
1 IJ - 9999 IJ
1 BZ - 9999 BZ
1 JZ - 9999 JZ
1 SZ - 9999 SZ
AIJ 1 - AIJ 9999
BIJ 1 - BIJ 9999
...
YIJ 1 - YIJ 9999
ABZ 1 - ABZ 9999
BBZ 1 - BBZ 9999
...
Currently at TBZ.
The numbers for all the counties are issued centrally from Coleraine. Up to a few years
ago the first 100 numbers in each series were held back for resale; now it's the first
1000 as well as some other desirable numbers.
As well as KIL in Fermanagh, another number sequence not to be issued was NAZ in Belfast,
and I've never seen any IOI or OOI numbers (Belfast), so I presume they weren't issued
either.
Fermanagh is a sparsely populated area and as you can see only ever needed to be issued
with one two-letter index mark (IL). Fermanagh numbers seem to be getting very popular
in other places and it looks like it will have to move on to a new system before any
of the other counties.
A big difference between the system here in Northern Ireland and in Great Britain and the
rest of Ireland is that vehicles being re-registered are given current-issue numbers,
which makes them seem much newer than they really are. This is because although the
numbers don't have a year attached to them, they are issued in sequence and it isn't
too difficult to tell roughly how old a vehicle is from looking at its number (for your
own county anyway- different volumes of registrations in different counties mean the
system moves at different rates).