Unfortunately, some important records are no longer in existence mainly due to:
Fire in the Four Courts, Dublin, 1922.

The destruction of the Public Record Office of Ireland in the Four Courts, Dublin (as the result of a fire during civil disturbance in 1922) left a considerable gap in the archival heritage of Northern Ireland. Many important records were lost, including:
The census returns from 1821 to 1851
A small number of volumes survived, covering parts of Co. Fermanagh and Cavan in 1821; parts of Co. Londonderry and for Killeshandra Parish in County Cavan for 1831; and for parts of County Antrim for 1851. These are available in PRONI under the main reference number MIC/5A. Extracts from the 1841 and 1851 census returns can be found in the Old Age Pension books - those for Northern Ireland are in PRONI under the main reference number T/550 see Your Family Tree Leaflet 5 - Census Records (19th Century) (27KB) for further details.

Pre-1858 original wills, administration bonds and marriage licence bonds
Although the original wills, administration bonds and marriage licence bonds were destroyed, indexes survived in manuscript and printed form. Those for the dioceses covering Northern Ireland are available in PRONI - it is therefore possible to extract some details about individuals from these indices. Copies of many destroyed wills can also be found in various privately deposited archives.

Church of Ireland parish records
The records of 1,006 Church of Ireland parishes, originally deposited in the Public Record Office of Ireland in Dublin, were largely destroyed in 1922. However, most of those relating to Northern Ireland (and several from the Republic of Ireland) that survived are available in PRONI either on microfilm or in original form. Those surviving for the majority of parishes in the Republic of Ireland can be accessed at the National Archives of Ireland in Dublin.

The destruction of census records by Government order
Census returns covering the whole island of Ireland for the years 1861-1891 were destroyed by order of the Government on grounds of confidentiality.

RESEARCHING YOUR IRISH ROOTS

Hopefully these sites will offer you a pathway to finding your Irish Ancestor.
P R O N I Public Records Office of Northern Ireland
GENUKI: Ireland for information related to all of Ireland
CENSUS
Irish Genealogy Exploring your Irish family history, step-by-step

Church records in Ireland.
The church records preserve details of the baptisms, marriages and burials which took place within a particular parish, church or congregation and were usually compiled by the relevant clergyman.

There is a great degree of variation in the level of detail contained within these records. Indeed over a period of one hundred years or more there can be considerable variation even within a single parish, church or congregation. In general, baptism records record the date of the baptism; the names of the child, the parents and the names of the child?s sponsors or godparents. The family address and the name of the clergyman may also be recorded.

Marriage records generally record the date of the marriage, the names of the spouses and witnesses. Other information such as the names of the spouse?s parents, residences of the spouses, ages, occupations and the name of the clergyman may also be recorded.