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IreGaz is an accurate record of most of the administrative divisions of Ireland: extremely accurate (probably >99.9%) in respect of Townlands, Baronies, Counties, Provinces and Poor Law Unions (PLUs); and fairly accurate (probably >99%) in respect of Electoral Divisions (EDs). Accuracy is tested against various area-totals in Townland indexes and Census summaries (1851-1911), so some placenames in some years show zero area in IreGaz: these placenames may have been unaccounted at the date in question, either through oversight, or because they did not exist at the time.
Sadly it is not possible to achieve the same accuracy in the case of Registrar's Districts, originally formed as Dispensary Districts. See a discussion here. And in the 1860s, townlands in some urban areas disappeared from view, when Townships were established in accordance with the Towns Improvement (Ireland) Act 1854.
There are quite a few instances of error in the original documents, not all of which can be resolved satisfactorily. In cases of conflict, IreGaz generally prefers OS and/or Valuation sources to Census sources.
Such time-saving calculations do not significantly affect the accuracy of ED areas, because the area of small islands is such a tiny percentage of even the smallest ED; but they make it difficult for IreGaz to allocate each and every small island to its correct ED. The 1901 Census summary is the earliest to contain tables allocating groups of small islands to EDs; IreGaz works backwards from these 1901 tables to minimise errors of allocation in earlier years.
There is a more drastic problem with small islands, and even with some fairly large ones: between one survey and the next, islands may come into existence, change size, or disappear (sometimes by being absorbed into another island or into adjacent mainland). In its inner workings, IreGaz keeps track of identifiable changes; and extra notes in the listings try to explain some of them.
To see how IreGaz deals with these various problems concerning islands, here is a worked example.