New administrative areas resulting from the Act came into effect on 18 April 1899. Preparation for the changes had been happening for some years, with alterations to the names and boundaries of Registrar's Districts (RDs).

There were wide-ranging effects on the designation of administrative areas, most noticeable in urban areas; and a number of rural boundaries and names were also changed. The Census summary of 1901 found it difficult to keep pace with these changes, the summary contains a significant number of typographical and mathematical errors, and in some urban areas it bluntly states that acreage figures were not available.

It thus becomes difficult to follow former townlands through their urban transformations, especially when updated measurements are adopted for an area, or when the 1901 Census summary makes no reference to earlier administrative areas. When measurements have been updated, IreGaz has been obliged to reduce reference to former townlands in urban areas; this is at its most drastic in the cities of Dublin and Limerick, where former townlands cannot be reconciled with newer designated areas. Where measurements remain unchanged, IreGaz displays an equal and opposite incompatibility: in Waterford and Sligo, for example, IreGaz retains references to former townlands, but they are not cross-referenced to new urban District Electoral Divisions (EDs).

In time, new sources of information may be found which will aid further progress. But from current sources IreGaz cannot develop its unique progressive national gazetteer beyond 1901.