Civil or state registration of non Roman Catholic (RC) marriages began in 1845, while registration for RC births, marriages and deaths, and for non RC births and deaths, began in 1864. The term 'non RC' refers to members of the Church of Ireland (Anglicans), Presbyterians, Methodists, Baptists, Quakers, Huguenots, Lutherans and a variety of other belief systems with followers in Ireland, from the 17th century onwards.
When civil registration began, the authorities decided to collect this new information based on existing administrative divisions. Consequently all births, marriages and deaths were registered according to an administrative division known as a ‘Poor Law Union’ (PLU) or ‘Superintendent Registrar’s District' (SRD).
To search for a birth, marriage or death record for your ancestor, you need to know the ‘PLU’ or ‘SRD’ in which the event would have been registered. There are approx. 163 PLU’s in Ireland, many of which cross county boundaries.
Once a birth, marriage or death occurred, it was a legal obligation for the event to be registered with the civil authorities, and full details supplied within three months for births (but less so for marriages and deaths). The onus for registration (for births and deaths) fell to the public (parents for births and the officiating clergyman for deaths, while registration of marriages was generally also the responsibility of the officiating clergyman).
Non-registration of a birth, marriage or death within the permitted time resulted in a fine. Consequently, to avoid payment of the fine, the birth, marriage or death date might in some cases be ‘moved’ to suit the date of registration! For this reason, where complete accuracy is required, it is always wise to check the relevant church records as the church did not impose such fines, and as such they are generally considered to be a more reliable indicator of the true date on which the event took place.
Despite the imposition of a fine, this did not deter a certain percentage of the population failing to register a birth, marriage or a death. Genealogists have estimated that about 10-15% of marriages and births simply do not appear in the registers.
 Tracing Your Irish Ancestors, John Grenham 2012, 7.