First, the bad news (not that it’s news: if you’ve been pursuing Irish genealogy at a level beyond that of absolute beginner, you already know about the sad loss of the nineteenth-century Irish census returns):
There are no surviving census returns for 1891, 1881, 1871, and 1861. And there are very, very few surviving census returns for 1851, 1841, 1831, and 1821.
Yes, I know: it is to weep. John Grenham explains the sorry situation at his Irish Roots column.
And now for a bit of good news (and this really is a new and welcome development):
The surviving pre-1901 census records (and to reiterate: most pre-1901 Irish census returns did not survive) are now available online, and free of charge, at the National Archives of Ireland’s website (census.nationalarchives.ie).
Shaun O’Connor at Ottawa Family Tree has a nice post on the topic, with a summary of what’s available.