The first Irish census was taken in 1813 but had so many flaws that the records were deemed useless and destroyed. A second, more successful, attempt was made in 1821 and a census was subsequently held every decade until 1911.
The information contained in these census records varied according to the year but always included at least the names of all individuals, their ages and their relationships to their head of household, plus some basic data about their land or home.
You could reasonably expect that after such extensive record-taking (taken 20 years before all-names censuses in England, Scotland and Wales, and 30 years before the USA), ancestral search through the 19th century would be a breeze.
Sadly, that is not the case.
The original census records for 1861 and 1871 were destroyed shortly after they are collected, and for some inexplicable reason (possibly for something as prosaic as a need to create additional storage space), the records for 1881 to 1891 were pulped, by government order, during the First World War.
Just a few years later, in 1922, an explosion and subsequent fire at the Public Records Office in Dublin destroyed most of the four censuses taken from 1821 to 1851. Only a few fragments of these censuses survive.
means that the Irish censuses of 1901 and 1911 are the only complete sets
available for your ancestor search. These have now been digitised and released,
Because virtually all of Ireland's 19th century census records were destroyed, family historians must instead rely on other genealogical sources for that period. These are collectively known as 'census substitutes'.
The most useful are land records (especially Griffith's Valuation), religious censuses, school registers, old-age pension applications and other miscellaneous lists of names such as trade directories.
Before you start looking into those alternatives, though, take a look to see if any of the fragments of Ireland's earliest censuses are worth pursuing. Their potential value will depend largely on whether or not you know your ancestors' place of origin.
An Irish census wasn't taken in 1921 due to the civil war which was raging throughout the island at that time. However, one was taken in 1926 in both north and south.
wartime National Register: Taken just weeks after Britain declared war on
Germany, this register of the population of Northern Ireland was a census in
all but name. (No such register was taken in the Republic of Ireland.)
The paperwork survives and access to it is currently controlled by Freedom of Information laws. If you have the names and addresses of individuals living in the six counties, you can make an FOI application, free, via PRONI.
The FOI procedure is not in the slightest difficult, so don't be put off.