Among the vast majority of the population, the range of first names in use in the nineteenth century was severely limited. John, Patrick, Michael, Mary and Bridget occur with almost unbelievable frequency in all parts of the country. Combined with the intensely local nature of surnames, reflecting the earlier tribal areas of the country, this can present intense difficulties when using the indexes.
For example, a single quarter of 1881, from January to March, might contain twenty or more John (O')Reilly (or Riley) registrations, all in the same registration district of Co. Cavan. A further obstacle is the fact that it is very rare for more than one first name to be registered. Thus someone known to the family as John James (O)Reilly will almost certainly appear in the index as a simple John. It is of course possible to purchase photocopies of all of the original register entries, but unless some other piece of information such as the parents' names or the townland address can be used to cross check, it will almost certainly not be possible to identify which, if any, of the original register entries is the relevant one.
This uncertainty is compounded still further by the persistent imprecision regarding ages and dates of birth, which means that over the seven or eight year period when the relevant birth could have taken place, there might be fifty or sixty births of the same name in the one county.
One way to surmount the problem, if the precise district is known, is to examine the original registers themselves to build a picture of all families in which the relevant name occurs. As already mentioned, the originals are still kept in the local registrars' offices. Although the situation varies from district to district, people visiting the offices in person are usually allowed to examine the original books. The relevant addresses can be found in local telephone directories, under the Health Board. Even in the country-wide Indexes, however, despite all of the problems there are a number of ways in which the births indexes can be used successfully, by narrowing the area and period to be searched with information obtained from other sources.