There is only a fragment of the pre-1901 Irish censuses that survived the depredations of man and fire. Now the Irish Research Group of the Ottawa Branch of the Ontario Genealogical Society have placed online a transcript of the surviving parts of the 1851 Irish census and supporting documentation, all accessible from here.
The records are mainly for part of County Antrim, located in the extreme north-east of the island, include part or all of 14 civil parishes from various parts of the county (almost 29,000 records).
Added are a 1853 occupant summary of the 1851 census of County Cork, which is located in the south-west part of the island, includes nearly 4,000 records from four adjacent parishes: Kilcrumper, Kilworth, Leitrim and Macroney.