The collection, which covers all 32* of the counties of the island of Ireland, is the first in a series of National Archives records of genealogical interest to be digitised by the Genealogical Society of Utah ie FamilySearch in partnership with the National Archives and be placed online with free access.
As with the existing 1901 and 1911 census website, the interface is clutter free and it's easy to find your way around. You can search by individual – using surname and locations – or you can browse townlands to view neighbourhoods.
When searching by location, bear in mind that the parishes used are civil or Church of Ireland parishes, not Roman Catholic parishes, which often had different names and boundaries.
Tithes were originally taxes on agricultural produce and were the main income of parish clergy. Occupiers of agricultural land were obliged to pay the tithe, regardless of their religion (and with the majority of the population being Roman Catholic, it was a particularly unpopular tax). Only rural areas were covered comprehensively and only heads of households were named.
The Tithe Applotment Books provide the earliest surviving national list of occupiers of land. The original books for the 26 counties of the Republic are held by NAI while those for the six counties of Northern Ireland are held by PRONI.
The collection can be accessed through the NAI's new genealogy portal here.
*UPDATE: Only it doesn't cover 32 counties. Antrim and Derry are missing. I've emailed the project team and will let you know the score as soon as I hear back.
*UPDATE – 9 November: Response from the project team: 'The books for Northern Ireland are held by PRONI. We are in discussion with them regarding possibly adding them to our site and giving them a copy of the whole thing.'
UPDATE – 12 November: Missing parishes, Northern Ireland, and corrections.