The Composition Act of 1823 specified that tithes due to the Established Church, the Church of Ireland, which had hitherto been payable in kind - in produce grown - should now be paid in money. As a result, it was necessary to carry out a valuation of the entire country, civil parish by civil parish, to determine how much would be payable by each landholder.

This was done over the ensuing 15 years, up to the abolition of tithes in 1838. Not surprisingly, tithes were fiercely resented by those who were not members of the Church of Ireland, and all the more because the tax was not payable on all land and this produced spectacular inequalities. In Munster, for instance, tithes were payable on potato patches but not on grassland, with the result that the poorest had to pay most.

The exemptions also mean that the Tithe Books are not comprehensive. Apart from the fact that they omit entirely anyone not in occupation of land, certain categories of land, varying from area to area, are simply passed over in silence. They are not a full list of householders. Nonetheless, they do constitute the only countrywide survey for the period, and are valuable precisely because the heaviest burden of tithes fell on the poorest, for whom few other records survive.

From a genealogical point of view, the information recorded in the Tithe Books is quite basic, consisting typically of:

In addition, many books also record the landlord's name and an assessment of the economic productivity of the land. The tax was based on the average price of wheat and oats over the seven years up to 1823 and was imposed at different rates depending on the quality of the land.

An organised campaign of resistance to the payment of Tithes, the so-called Tithe War, culminated in 1831 with large-scale refusals to pay the tax. To apply for compensation for the resultant loss of income, local Church of Ireland clergymen had to produce lists of people liable for tithes unpaid - the 'Tithe Defaulters'.

The lists can provide a fuller picture of tithe-payers than the original Tithe Book and can be useful to cross-check against the book, especially if it dates from before 1831. 127 of these lists survive, in the NAI Chief Secretary's Office, Official Papers series. They relate principally to counties Kilkenny and Tipperary, with some coverage also of counties Carlow, Cork, Kerry, Laois, Limerick, Louth, Meath, Offaly, Waterford and Wexford.

A full list was published in The Irish Genealogist (Vol. 8, No. 1, 1990). County-by-county microfiche indexes have been produced by Data Tree Publishing in Australia. These are available at the National Library of Ireland.

Microfilm copies of the Tithe Books are available in the National Archives and the National Library. Those for the nine counties of Ulster are available in the Public Record Office of Northern Ireland.

The usefulness of the Tithe Books can vary enormously, depending on the nature of the research. Since only a name is given, with no indication of family relationships, any conclusions drawn are inevitably somewhat speculative. However, for parishes where registers do not begin until after 1850, they are often the only early records surviving. They can provide valuable circumstantial evidence, especially where a holding passed from father to son in the period between the Tithe survey and Griffith's Valuation. The surnames in the books have been roughly indexed in the National Library Index of Surnames.