Dublin postal districts are used by Ireland's postal service, known as An Post, to sort mail in the Dublin area, This system is similar to that used in London and other UK cities before the advent of the postcode. The postal district appears with one or two digits appearing at the end of addresses, e.g:
British Embassy 29 Merrion Road Ballsbridge Dublin 4
Odd numbers are generally used for addresses on the Northside of the River Liffey, whilst even numbers are on addresses on the Southside. The lowest numbers are near the centre (for example, Dublin 1 refers to the North city centre) and the the highest numbers are at the periphery (for example, Dublin 24 refers to the suburb of Tallaght). The outer area of County Dublin does not have postcodes assigned.
The term 'Dublin 4' is used colloquially in the Republic to refer to the country's middle class liberal establishment. In 1985, Dublin 6 was split, with some areas, such as Templeogue, Kimmage and Terenure becoming part of a new district, called Dublin 6W:
These numbers appear on most street signs in Dublin, next to the name of the street in English and Irish, hence the 'MERRION ROAD' (Bothair Muirfean) street sign will display the digit '4'. In Cork, there are also numbered districts, eg: the 'PATRICK STREET' (Sraid Phadraig) sign will display the digit '1', but these are not encountered in postal addresses.
Similar schemes were used in cities in other European countries until they adopted a national postal code system in the 1960s and '70s. However, Ireland did not follow suit, and An Post did not introduce automated sorting machines for mail until the 1990s. By then, the introduction of new technology, known as Optical Character Recognition (OCR), meant that machines could 'read' whole addresses as opposed to just postcodes. An Post argues that a national postcode system is unnecessary describing it as 'a 1960s solution to a 21st century problem'. Consequently, mail to addresses in the rest of the Republic does not require any digits after the address, eg:
World Wide Web Marketing Ltd. 4/5 High Street, Galway
However, the Communications Regulator in Ireland is considering the introduction of such a system in the light of the liberalisation of postal services, and the end of An Post's monopoly.