Sorry to be a party pooper, but as this is posted to County Down, I thought I should mention it.
The last all-Ireland census was in 1911. The 1926 census that it is hoped to release in 2016 is of the Irish Free State, which doesn't cover County Down nor the other 5 counties of Northern Ireland.
A census of Northern Ireland was carried out on 18/19 April 1926 and while the summary statistics are available, the important bits for us 'rooters' - the returns - were pulped during WWII. Another census tragedy.
There was no 1931 census of Northern Ireland.
A limited census of Northern Ireland was conducted in 1937, limited in terms of some information being omitted (eg. occupation). With rulings in recent years by the Information Commissioner, one can only hope that the normal 100 year rule will be waived and that we won't have to wait until 2038.
There was no 1941 census of Northern Ireland.
1939 National Registration information (name, sex, age, marital status, military membership) is held by PRONI and can be released under FOI, but you have to be able to provide an exact address and demonstrate that the person you're seeking is deceased.
The next full census of Northern Ireland was in 1951.
Hope this clarifies.
