Birth indexes there go from January 1864 to December 1921, including the six counties in the North. Prior to 1864, one must use Church records. The registration district is needed in order to obtain the actual record. In 1922 the indexes are for records in the Republic of Ireland, while the six counties of Derry, Antrim, Armagh, Fermanagh, a Tyrone are in the Public Records Office of Northern Ireland (PRONI) in Belfast. Each county has a registry; check the GRO Web site for information.
Death indexes are registered in Ireland for the same years as birth indexes, again including the six counties in the North. The Death Index also lists the age at death. As of 1922, the same situation applies as for birth records.
Marriage indexes are registered in Ireland for the same years, again including the counties of the North, and the same rule applies for records after 1922.
For the years 1864 to 1877 the indexes are alphabetical by surname, and then the given name. Each index covers a single year for the entire country. One must know the registration district to determine the right record. In 1878 the indexes are divided into 4 quarters - January-March, April-June, July-September, and October-December. Look for ancestors in each quarter of the year and again know the registration district. In 1903 the birth index covers the entire year, and also gives the mother's maiden name.