The order followed in the indexes is strictly alphabetical, but it is always necessary to keep possible variants of the surname in mind. In the late nineteenth century, when a large majority of the population was illiterate, the precise spelling of their surnames was a matter of indifference to most people. Thus members of the same family may be registered as, for example, Kilfoyle, Gilfoyle, or Guilfoile. The question of variants is particularly important for names beginning with O or Mac. Until the start of the Gaelic revival at end of the last century, these prefixes were treated as entirely optional and, in the case of the O's particularly, more often omitted than included. Until well into the twentieth century, for instance, almost all of the O Briens are recorded under Brien or Bryan. Before starting a search in the indexes, therefore, it is essential to have as clear an idea as possible of the variants to be checked. Otherwise it may be necessary to cover the same period as many as three or four times.