The basic geographical unit used in carrying out both the 1901 and 1911 censuses is the District Electoral Division, a subdivision of the county, used, as the name implies, for electoral purposes. To use the returns, ideally the relevant street or townland should be known. The 1901 Townlands Index, based on the census returns, supplies the name and number of the DED in which the townland is situated.

County-by-county volumes, on open shelves in the National Archives Reading Room, contain the DEDs in numerical order for both 1901 and 1911, giving the name and number of each of the townlands they contain.

To order the returns for a specific townland, it is necessary to supply the name of the county, the number of the DED and the number of the townland, as given in these volumes. For the cities of Belfast, Cork, Dublin and Limerick separate street indexes have been compiled and are also on open shelves in the Reading Room. Again, each street or part of a street is numbered and these numbers are necessary to order specific returns.

Between 1901 and 1911, some changes took place in the District Electoral Divisions, and their numbering is different in some cases. There is no separate townlands index for 1911, but the changes are minor, so that a DED numbered 100 in 1901 may be 103 in 1911. It can be found simply by checking the divisions above and below 100 in the 1911 volume for the county. The same procedure is necessary for the copies of the two censuses held by the LDS Family History Library.

Both the 1901 and 1911 censuses are available on microfilm. In each case, all the returns for a townland or street are grouped together and preceded by an enumerator's abstract that gives the details of the houses and lists the names of the heads of households. These lists can be very useful if the precise townland or street is not known and it is necessary to search a large area, checking all households of a particular surname. Such a procedure is, however, less precise than a check of each of the returns themselves.

The order in which the returns were microfilmed can sometimes be problematic, with townlands or DEDs out of sequence. One problem that can arise in searching a large area is the difficulty of translating from the earlier geographical division of a parish. For instance, the relevant District Electoral Divisions cut across the earlier boundaries.

The most straightforward, though cumbersome, way to cover a large area is to take all of the townlands, in particular civil parishes, and check their District Electoral Divisions in the 1901 Townlands Index. The 1841 Townlands Index - also known as Addenda to the 1841 Census - is available on request from the National Archives Reading Room staff or in the National Library in Dublin. It organises townlands alphabetically within civil parishes. (The reference number is Ir 310 c 1.)

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