Original Allocations — Ireland
Ireland in its entirety was part of the United Kingdom when the original allocations were made in 1903, and therefore all areas of the island received allocations under the system. All index marks with the letter 'I' were reserved for Irish index marks, and to avoid confusion there were at that point no allocations that combined the letter 'I' with those reserved for Scottish plates, i.e. 'G', 'S' or 'V'. So there was no 'GI' or 'IV', for example. The initial allocations were made to the counties in alphabetical order (as they were at the time, hence "King's County" instead of "Offaly" and "Queen's County" instead of "Laois"), and thereafter to the cities.
Once these were all allocated and some were beginning to be exhausted, the previously unused 'Z' series was also allocated to Ireland. By the time this happened Ireland had been partitioned, and it was decided that index marks with a 'Z' as the first letter would be allocated to the Irish Free State (now the Republic of Ireland) and those with the 'Z' as the second letter would go to Northern Ireland. As with the 'I' series, no combinations with 'G', 'S' or 'V' were allowed, although these were issued later as marks began to run out, both in Northern Ireland and the Republic.
In Northern Ireland, the two-letter marks were reversed as in the rest of the UK in the late 1950s and early 1960s, but when a 7-symbol system was introduced it was not by the addition of a year-letter suffix but by the addition of a fourth number, e.g. [AOI 1234]. In the Republic, the 6-symbol system was used, both forwards and reversed, until the new system was introduced in 1987.
To be continued.