Because the 1926 census for Northern Ireland was used for waste paper in World War II, the first census since 1911 that survives for the city and county of Londonderry is that of 1937, and this will be available for inspection in the year 2038 (unless the 100-year closure rule is waived before then). This means that each annual edition of Derry Almanac is the closest surviving census document for Derry city in the period from 1912 to 1936.
From 1868 right through to 1949 inclusive, each annual edition of the Derry Almanac and Directory contained a Street Directory, where heads of households were identified against their street address in Derry city. The recording of house numbers, against each householder, first appeared in the Almanac of 1897.
This publication by Irish genealogist Brian Mitchell lists inhabitants, in alphabetical order by surname, in Derry city in 1921. As transcribed here, the work contains five fields: Surname of Head of Household, First Name of Head of Household, Street Address, House Number, and Page Number of the listing in Derry Almanac. In all, the Derry Almanac and directory of 1921 name 8,288 heads of household in Derry city.
Especially significant about the 1921 directory for Derry researchers are its references to Derry inhabitants amidst the turmoil of (1) the June 1920 six-day “civil war” between Nationalist and Unionist supporters in Derry city, which left 23 people dead and many wounded, and (2) the subsequent Irish War of Independence and Anglo-Irish Treaty, from 21 January 1919 to 6 December 1921. In effect, this work from Brian Mitchell is the closest thing we have to a census for Derry during this period of upheaval.