In the 18th and 19th centuries the vast majority of the Irish population lived as small tenant farmers on large estates owned for the most part by English or Anglo-Irish landlords. The administration of these estates inevitably produced large quantities of records such as maps tenants' lists, rentals, account books and lease books. Over the course of the 20th century, as the estates have been broken up and sold off, many have found their way into public collections and make up a largely unexplored source of genealogical information.

There are, however, good reasons for their being unexplored. First, it was quite rare for a large landowner to have individual rental or lease agreements with the huge numbers of small tenants on his land. Instead, he would let a significant area to a middleman, who would then sublet to others, who might in turn rent out parts to the smallest tenants. It is very rare for estate records to document the smallest landholders, since most of these had no right of tenure in any case, being simply tenants 'at will'.

A related problem is the question of access. The estate records in the two major Dublin repositories, The National Archives and The National Library, are not catalogued in detail. The only comprehensive guide is given in Richard Hayes' Manuscript Sources for the Study of Irish Civilisation and its supplements, copies of which can be found in the National Library and National Archives.

This catalogues the records by landlord's name and by county, with entries such as NL Ms 3185 Rent Roll of Lord Cremorne's estate in County Armagh, 1797. Hayes gives no more detail of the areas of the county covered, and it can be difficult to ascertain from the Tithe Books or Griffith's just who the landlord was. Griffith's only supplies the name of the immediate lessor. The holdings of the Public Record Office of Northern Ireland are catalogued more comprehensively, but still do not relate the papers to the precise areas covered. Again, it is necessary to know the landlord's name. In addition, many of the collections in the National Library have still not been catalogued at all and remain completely inaccessible.

The largest single collection in The National Archives is the Landed Estate Court records, also known as the Encumbered Estate Courts, which are not catalogued in Hayes. The Court was set up to facilitate the sale of estates whose owners could not invest enough to make them productive, and between 1849 and 1857 oversaw the sale of more than 3,000 Irish estates. Its records contain many rentals and maps drawn up for the sales, but are so close in time to Griffith's they are of limited use except in very particular circumstances. The National Archives have an index to the townlands covered by the records.

Despite all the problems, research in estate records can be very rewarding, especially for the period before the major 19th-century surveys. To take one example, the rent rolls of the estate of Charles O'Hara in Counties Sligo and Leitrim, which date from c. 1775, record a large number of leases to smaller tenants and supply the lives named in the leases, often specifying family relationships. It must be emphasised, however, that information of this quality is rare; the majority of the rentals and tenants' lists surviving only give details of major tenants.

The Public Record Office of Northern Ireland also holds the papers of a large number of estates, many of which cover areas now in the Republic of Ireland, Their web site www.proni.gov.uk/ gives details of the collections.

A more detailed guide to the dates, areas covered and class of tenants recorded in the estate papers of the National Library and National Archives is in the process of preparation by the National Library, in association with the Irish Genealogical Society of Minnesota. To date, Counties Armagh, Cavan, Cork, Donegal, Fermanagh, Kerry, Kildare, Leitrim, Limerick, Galway, Mayo, Monaghan, Roscommon, Sligo, Tyrone and Waterford have been covered.