After the coming of the Reformation to Ireland in the sixteenth century, the parish structures of the Catholic Church and the Church of Ireland diverged. In general, the Church of Ireland retained the older medieval parochial divisions, which were also used for administrative purposes by the secular authorities.
Thus civil parishes, the basic geographical units in early censuses, tax records and land surveys, are almost identical to Church of Ireland parishes. The Catholic Church, on the other hand, weakened by the confiscation of its assets and the restrictions on its clergy, had to create larger and less convenient parishes.
In some ways, however, this weakness produced more flexibility, allowing parishes to be centered on new, growing population centres, and, in the nineteenth century, permitting the creation of new parishes to accommodate this growth in population. The differences in the parish structures of the two churches are reflected in their records.
Even allowing for the fact that members of the Church of Ireland were almost always a small minority of the total population, the records of each parish are proportionally less extensive than Catholic records, covering a smaller area, and are thus relatively easy to search in detail.
Catholic records, by contrast, cover the majority of the population
and a much larger geographical area, and as a result can be
very time-consuming to search in detail. The creation of new
Catholic parishes in the nineteenth century can also mean that
the registers relevant to a particular area may be split between
two parishes. Both Catholic and Church of Ireland parishes are
organized on the diocesan basis first laid out in the Synod
of Kells in the Middle Ages, and remain almost identical, although
the Catholic system has amalgamated some of the small medieval