For many people researching their Irish genealogy, the destruction of the Dublin record office in 1922 during the civil war, has left huge gaping holes in what information is actually available.
The Public Record Office of Ireland, at Four Courts, Dublin had in fact been occupied 
during the Easter Rising, however remarkably no serious damage had been done to the archives, famously the only record had actually been destroyed, that being the 19th century will of an Irish soldier. However six years later, on the 3rd June 1922, the record office was blown up, with one enormous explosive device.
Eye witnesses recall that on that day, it literally rained archives in Dublin city. The smouldering remains of a thousand years of history, held in parish registers, census records, wills, court records, minute books, proclamations, etc. were scattering the streets. Some were retrieved and rescued, some handed in to the authorities, and other kept by private individuals.
The 1922 explosion and subsequent fire destroyed the national census records from 1821-1851, half the parochial registers of the Church of Ireland (Catholic registers were stored elsewhere), all pre 1900 court and government records, and centuries of wills, the irreplaceable loss will always hamper the efforts of individuals attempting to trace their Irish ancestry, however we should pay tribute to the Irish Genealogical Research Society formed in 1936, when its members set about attempting to find surviving records, rescued, but still held in private hands. Through their efforts much material was repatriated into the National Archives, and thanks to their tireless efforts, records previously thought lost have now been preserved for the future. Their website linked above, is always a good start for anybody wanting to find out more about their Irish ancestors.
Catholic Family History 