When carrying out research in all three areas, a large dose of scepticism is necessary with regard to the dates of births, marriages and deaths reported by family members before 1900. This is especially true for births. The ages given in census returns, for example, are almost always inaccurate and round figures - 50. 60, 70 etc. - should be treated with particular caution. The actual date of birth is almost always well before the one reported, sometimes by as much as fifteen years. Why this should be is a matter for speculation, but vanity and mendacity are probably not to blame. Until quite recently very few people actually knew their precise date of birth. And as most people don't feel their age, after middle age at least, a guess will usually produce an underestimate. Whatever the explanation, charitable or otherwise, it is always wiser to search a range before the reported date, rather than after it.
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From 1864 to 1877 the indexes consist of a single yearly volume in each category - births, marriages and deaths - covering the entire country, and recording all names in a straightforward alphabetical arrangement. The same arrangement also applies to the non-Catholic marriages registered from 1845.
From 1878, the yearly volume is divided into four quarters, with each quarter covering three months and indexed separately.
This means that a search for a name in, for example, the 1877 births index involves looking in one place in the index, while it is necessary to check four different places in the 1878 index, one in each of the four quarters. From 1900, in the case of births only, the indexes once again cover the entire year, and only from this year also supply the mother's maiden surname.
The online indexes at IrishGenealogy.ie are good but they have peculiarities that should be kept in mind. All middle forenames are omitted, so an individual registered as 'John Francis Kelly' appears in the IrishGenealogy transcripts as 'John Kelly'. The treatment of the surname prefixes 'O' and 'Mc' is inconsistent, sometimes inserting a space after the prefix and sometimes not. Check under 'OConnor', 'O Connor' and 'O'Connor'. Every search covers all personal names in the record, so more results may be returned than expected. The variants lists are not complete and should be treated with caution. The marriage indexes do not always include both parties - search on both names if the record you want does not appear.
On the positive side, the searches use surname variants and forename variants by default, as well as allowing wild-card searches for both. The 'More Search Options' page allows precision and ingenuity. Mothers' maiden names are recorded in the index for all births from 1900, making it relatively easy to reconstruct family groups after that year. However, a search for birth records before 1900 that includes the mother's maiden name will return nothing. In general, the more specific the query, the less information is returned. It makes more sense in most cases to trawl broadly to start with and then begin to winnow the results. In order to purchase research copies of the pre-1878 death records not imaged on the site, the Group Registration ID or volume, page and registration district supplied in the index is necessary.
The FamilySearch version of the indexes is transcribed from the original printed volumes, which include middle names and treat 'O' and 'Mc' surnames consistently. They do not include mothers' maiden names until 1928, but they cover almost six decades of the twentieth century, coming much closer to the present than IrishGenealogy.ie.
In all three categories, each printed index entry (and hence the matching FamilySearch record) provides surname, first name, registration district, volume and page number. The death indexes also give the reported age at death. The 'volume and page number' simply make up the reference for the original register entry, necessary in order to identify it and, in the GRO research room, to obtain a printout of the full information given in that entry. More here on the remaining three items: