The most basic building blocks of genealogical research are surnames and places of origin. And like Irish surnames, Irish place-names have suffered extraordinary violence, mutating and deforming as they were forced out of Irish into English, mangled in written records, distorted in the folk-memories of migrants. A secure place-name identification can be a serious problem.
The standard tool has long been the 1851 Townlands Index, so called because it was produced as a guide to the 1851 census. However, its alphabetical listing of 64,000 or so townlands is not actually taken from the 1851 returns; the listing comes from the original Ordnance Survey of the 1830s and 1840s, the first wholesale standardisation of the spelling (in English) of townland names. The injury inflicted on Irish culture was grievous, but this listing remains very valuable. In particular, it was used in the creation of Griffith's Valuation. Identify a place in the 1851, and it will appear under identical spelling in the Valuation.
The 1851 Index is free online in three separate locations, at irishtimes.com/ancestor, thecore.com/seanruad and irish-place-names.com. The last is the most recent and the slickest, but suffers from over-simplification. Seanruad is the best known, but has quite a few omissions and can be inflexible to search. The version at irishtimes.com/ ancestor allows wild-card searches, a researcher's best friend, and also includes parish maps and street listings for Dublin, Belfast and Cork.
These three have now been joined at irishancestors.ie by the 1901 Townlands Index, used for the 1901 census returns, and very helpfully including District Electoral Divisions, the areas used for census collection after 1861.
Inevitable quibbles: the search interface does not allow wild-cards, and the presumption underlying the browse area that no two Irish DEDs or parishes have the same name is flawed. Try "Kilmore". But the arrival of the 1901 is still unequivocally welcome. The Irish Genealogical Research Society deserve whole- hearted thanks and congratulations.