1901 is the earliest surviving complete Census of Ireland.
Images of both 1901 and 1911 are available to view online (free of charge) at:
There were earlier censuses, but little material survives. A useful brief guide to surviving material is:
Sherry Irvine 'Results of Fire and Famine: Census Records in Ireland 1813-1911',
Ancestry Magazine, March/April 1999, vol 17 no 2
1831: The only surviving material is volumes of "retrospectively amended records" (Sherry Irvine) relating to County Londonderry.
This page was last updated 4 Sep 2016