Dublin postal districts are used by Ireland

Republic of Ireland

Ireland , described as the Republic of Ireland , is a sovereign state in Europe occupying approximately five-sixths of the island of the same name. Its capital is Dublin. Ireland, which had a population of 4.58 million in 2011, is a constitutional republic governed as a parliamentary democracy,...

's postal service, known as An Post

An Post

An Post is the State-owned provider of postal services in the Republic of Ireland. An Post provides a universal postal service to all parts of the country as a member of the Universal Postal Union...

, to sort mail in Dublin. The system is similar to that used in cities in other European countries until they adopted national postal code

Postal code

A postal code is a series of letters and/or digits appended to a postal address for the purpose of sorting mail. Once postal codes were introduced, other applications became possible.In February 2005, 117 of the 190 member countries of the Universal Postal Union had postal code systems...

 systems in the 1960s and '70s. These will be incorporated into the new national postcode system to be introduced in 2011.
The postal district system was introduced in 1917 by the British government, as a practical way to organise local postal distribution.