The inspiration and starting-point for IreGaz was the IreAtlas Townland database, produced many years ago by John Broderick (Sean Ruad). IreGaz is a tribute as well as a successor to his work. The IreAtlas database was based on the General Alphabetical Index to the Townlands and Towns, Parishes, and Baronies of Ireland, published in 1861 by the Registrar-General of Ireland. The 1861 Index was, in turn, based on three sources:
Like IreAtlas, IreGaz started with the 1861 Index. In addition, IreGaz has referred to the above three earlier sources, to enable detailed comparison and cross-checking. Such deep research would have been difficult without the images of original documents made available online free-of-charge by EPPI, Ask about Ireland, Ordnance Survey Ireland and the well-indexed histpop.
To find information about Electoral Divisions, IreGaz first consulted a later General Alphabetical Index to the Townlands and Towns, Parishes, and Baronies of Ireland, published in 1877, based on the 1871 Census of Ireland. As the 1877 Index seems to contain quite a few errors concerning Electoral Divisions, IreGaz has also consulted summaries of later Censuses, a Supplement to the Alphabetical Index to the Townlands and Towns, Parishes, and Baronies of Ireland (1882, based on the Census of 1881), and the General Topographical Index (1904, based on the Census of 1901).
Beware, however, there is a less reliable source for 1851: A Comparative View of the Census of Ireland in 1841-1851. This document was produced in 1852, the same year as the County Census summaries; but many area-totals differ from the County summaries, placenames are often mistranscribed, and typesetting is sometimes hurried (eg for New Ross PLU, many of the columns for roods and perches have become misaligned with their respective Electoral Division and acres). It seems clear that there was not enough proof-reading, and that the underlying data does not in detail match the County summaries. As 1851 County summaries can usually be verified by subsequent 1861 County summaries, it seems correct to treat as invalid the discrepancies in A Comparative View of the Census of Ireland in 1841-1851.
IreGaz has also made use of information contained in the facsimile portions of George B Handran's Townlands in Poor Law Unions (1997), the whole of which is made available as a download (payment required) by Eneclann. These facsimiles are lists printed by the General Register Office, Dublin: an all-Ireland list of Unions and their Registration Districts is dated May 1886, and uses data from the 1881 Census; and there are lists of later data for Belfast (June 1911), Cork (January 1901), Dublin North Union (June 1906) and Dublin South Union (June 1906).