The names and area-totals of Dispensary Districts were adjusted by a number of Orders during the 1850s (see various Annual Reports from 1853 onwards of the Commissioners for Administering the Laws for Relief of the Poor in Ireland under the Medical Charities Act). IreGaz has not yet attempted to identify many of these early changes.

The area-totals of Registrar's Districts (which adopted the areas of Dispensary Districts) are first recorded in the summary of the 1871 Census of Ireland. In Census data, not every Registrar's District (RD) is given an individual area-total, sometimes an aggregate total is given for a number of RDs.

An RD may contain many Electoral Divisions; or, at the opposite extreme, an Electoral Division may be split across two or more RDs. The Handran source is helpful by showing the constituent parts of RDs in 1885; for earlier years, IreGaz has sometimes resorted to backward-extrapolation from 1885.

The boundaries of some RDs cut across a townland or urban parish. The areas of these split-townlands or split-parishes are very rarely recorded in Census data, so IreGaz has either calculated them from RD totals, or estimated them off the map.

Urban areas are often totalled in ways which make it difficult to reconcile one administrative area with another: a Parliamentary Borough differs from a Municipal Borough, and neither is necessarily the same as a Town/City for Census purposes; Ward boundaries rarely align with Parish boundaries; and none of the above boundaries necessarily align with the boundaries of RDs.

The above factors combine to make it difficult to define for a number of RDs (a) their constituent parts, and/or (b) their areas.

Two Electoral Divisions (Tivannagh, Co Roscommon & Killaraght, Co Sligo) within two rural RDs (Boyle No 1 & Boyle No 2) have defeated IreGaz's attempts at calculation, estimation and backward-extrapolation; their constituent townlands are therefore allocated to "Boyle No1, No 2". This is not too serious a matter, as the two RDs were combined in 1883.