Two surveys cover the entire country, the Tithe Applotment Books of c. 1823-38, and Griffith's Valuation, dating from 1848 to 1864. Both of these employ administrative divisions which are no longer in widespread use, and need some explanation. The smallest division, the townland, is the one which has proved most enduring. Loosely related to the ancient Gaelic "Bally betagh", and to other medieval land divisions such as ploughlands and quarters, townlands can vary enormously in size, from a single acre or less to several thousand acres.

There are more than 64,000 townlands in the country. They were used as the smallest geographical unit in both Tithe Survey and Griffith's, as well as census returns, and are still in use today. Anything from 5 to 30 townlands may be grouped together to form a civil parish. These are a legacy of the middle ages, pre-dating the formation of counties, and generally coextensive with the parishes of the Established Church, the Church of Ireland.

They are not to be confused with Roman Catholic parishes, which are usually much larger. In turn, civil parishes are collected together in baronies. Originally related to the tribal divisions, the "tuatha", of Celtic Ireland, these were multiplied and subdivided over the centuries up to their standardization in the 1500s, so that the current names represent a mixture of Gaelic, Anglo-Norman and English influences. A number of baronies, from five in County Leitrim to 22 in County Cork, go to make up the modern county. Baronies and civil parishes are no longer in use as administrative units.