All vital events in Ireland are registered in the Registration District where it took place.  A point of note, marriages, while an ancestor's town of origin may be in one Registrar's District, the local Catholic parish church, even though it might a short distance from his or her's residence, might well be in a different Registrar's District.  When searching for marriages, keep this in mind.

How do I find which district?

Probably the best resource to locate the Registrar's Distict where a particular townland resides is the 1851 edition of The General Alphabetical Index to the Townlands and Towns, Parishes and Baronies of Ireland. 
On line at:  http://www.seanruad.com

Townlands are the smallest unit of land division in Ireland.  The above index lists the location of each townland by county, barony, civil parish and Poor Law Union.  The boundaries of Poor Law Unions (PLUs) and Registrar's Districts are the same.

Volumes & Pages up to 1877

The original 130 districts were divided into 5 administrative areas.  The quarterly returns received by the Government Register Office (GRO) in Dublin were then bound up into the five areas (volumes), each registration district was assigned a certain volume.  So, each quarter, when the returns were due, five volumes were created.  When referring to the National Index, which is in straight A to Z sequence, by year, you will find volume and page columns. To determine which quarter the event took place, see the below table.
 

Knowing the volumes to determine which part of the year they refer to is not necessary after 1878 when the National Index was published in quarterly rather than yearly.

Northern Ireland

Up to 1922, all of Northern Ireland's records are included in Ireland's civil registration.  With the creation of Northern Ireland in 1922, an amount of realignment had to take place in Volumes 2 and 3, while Volume 1, which was almost all Northern Ireland, was abolished.  Check the following table to see which Registrar's Districts were effected.

The information recorded on a death record is:  date and place of death; given name(s) and surname; sex; marital status; age; occupation; cause of death; the name and address of informant, and date of registration and signature of the Registrar.  However, where the deceased died in an institution (hospital, workhouse, sanatorium) the home address might not be recorded, and the age might be inaccurate as might the marital status, since the info was probably given by the institution's employee who may not have been very familiar with the person.

1864-1874

It should be noted that in the first ten years of registration (1864-1874) a very good number of events went unregistered.  A more went unregistered in the west of Ireland than in any other part of the country.  The greatest compliance was in urban areas.

ConnorsGenealogy Links

Table of registrar's districts with volume# and county

Registrar's Districts Map

LDS film number for Ireland Birth Index 1864-1921

LDS film numbers for Ireland Birth Records 1864-1955

LDS film numbers for Ireland Death Index 1864-1921

LDS film number for Ireland Death Records 1864-1870

References:

Irish Civil Registration-Where do I start?  Excerpts from a pamphlet I picked up in Dublin in 2001.  By Eileen M Ó Dúill, & Steven C. ffeary-Smyrl.

Townlands in Poor Law Unions, A Reprint of Poor Law Union Pamphlets of the General Registrar's Office, by George B Handran, CG, 1997