The present- day vehicle registration scheme in the British Isles started with the Motor Car Act of 1903 (effective   01/01/04 - the fee was one pound and the speed limit fixed at 20 mph).

From looking at the table it appears that the scheme was initially quite simple. The letter 'I' was assigned to Ireland and the 32 counties were then given 33  (because Tipperary is divided into the North & South Ridings) combinations using the letter 'I' in alphabetical order (excluding those with G,I,Q,S & V) thus:

IA = Antrim,
IB = Armagh......
IZ = Mayo,
then
AI = Meath......
NI = Wicklow
and then
OI - WI
were allocated alphabetically to the six major cities.

It would appear that the remaining letters and the 'Z's were divided at a later date between the Irish Free State/ Republic of Ireland (Z - ZZ) and Northern Ireland (AZ - YZ).

A three figure (or four in NI) number was then added to make the 'number plate' and if the number series ran out then a letter was prefixed to the 2 letter code.

Occasionally the sequence was reversed ie
123 ABC.

Although not strictly relevant, in September 2001, the DVLA (Driver & Vehicle Licensing Authority) introduced a replacement to the 1904 registration mark scheme for new vehicle registration in Great Britain which, after the introduction of the new plates in the Republic of Ireland on 1st January 1987, now leaves only Northern Ireland, the Channel Islands and the Isle of Man as the locations where the 1904 Act is still in force. 

First published: 29/08/98
Last amended: 15/09/03