The 1901 and 1911 Censuses are the only complete censuses to survive for the whole island of Ireland, and both are freely searchable online. For every household they record names, ages, relationships, religion, occupation, literacy and birthplace — and, in 1911, the number of years married and the number of children born and still living.
The earlier 19th-century censuses fared badly: the 1821–1851 returns were largely destroyed in the 1922 Public Record Office fire, surviving only in fragments, while the 1861–1891 returns were deliberately destroyed by the government of the day. We make the most of what survives, and of the census substitutes that help fill the gaps.