Ireland's Valuation office conducted its first survey of property ownership in Ireland from 1848 to 1864. The surveyors started this enormous task in the very south of the country so the valuations for Cork or Waterford date from 1848 while the Province of Ulster was last to be valued between 1858 and 1864.
This survey became known as "Griffiths Valuation" from Sir Richard Griffith who was the director of the Valuation Office at that time. The survey was used to determine the amount of tax each person should pay towards the support of the poor within their Poor Law Union. This involved determining the value of all privately held lands and buildings in rural as well as urban areas to figure the rate at which each unit of property could be rented year after year.
The resulting survey was arranged by barony and civil parish with an index to the townlands appearing in each volume.
Griffith's Valuation can be used as an excellent census substitute for the years after the Great Famine as all census records prior to 1901 were mostly destroyed by a fire in the early part of the 20th century.
Use of the online index is simple: go to the web page, enter the surname of interest and the county and then click on SEARCH. A second or two later, a list of all the matching entries will appear, such as this one record that I copied-and-pasted from the site:
First Name: Andrew
For common surnames, you can narrow the search by optionally specifying the townland and parish of interest. However, both the surname and county are always required. Therefore, you must have knowledge of which county to search before starting this unless the name is unusual and you are willing to conduct a separate search in each county looking for the person of interest.
This is a great resource but please remember that it is strictly an index, not a listing of all available information. It is somewhat similar to looking something up in the back-of-book index; once you find the information in the index you will want to turn to the referenced page to read all the details. In this case, once you find someone in the online index to Griffiths Valuation, you will want to find the original record to learn much more about your ancestor.
UlsterAncestry.com's free index to UlsterAncestry.com is available at http://www.ulsterancestry.com/search.html. While on the site you might want to look at the many other services available from this company.