There's been a problem accessing the pre-1901 census fragments at the National Archives of Ireland's Genealogy database at census.nationalarchives.ie this morning.
The National Archives has been notified and are working on putting it right.
In the meantime, if you need to gain access, try the FindMyPast.ie version (it's free but you need a registered account), which is working fine.
UPDATE, 16:40: Problem sorted! Full access restored.