The official term for 'townland' in Irish is baile fearainn (plural bailte fearainn); baile is the word for 'town' and fearann is 'land, territory, quarter', from a Proto-Indo-European root *wer- which is also related to the English word ware, a valuable commodity. (In Scottish Gaelic, the plural of the word baile is bailtean.) In the Isle of Man, the prefix "Balley-" is fairly common in farm names; the Manx for 'townland' is balley eirinagh, pl. baljyn eirinagh.
The term townland is a standardised form, often replacing earlier local terms such as tate (in Fermanagh and Monaghan), cartron (in Connacht) or ploughland. These terms represent a variety of native land divisions, varying in name from one part of the country to another, and forming a hierarchy of sizes.
10 acres - 1 Gneeve; 2 Gneeves - 1 Sessiagh; 3 Sessiaghs - 1 Tate or Ballyboe; 2 Ballyboes - 1 Ploughland, Seisreagh or Carrow; 4 Ploughlands - 1 Ballybetagh, or Townland; 30 Ballybetaghs - Triocha Céad or Barony.
(A complicating factor was that in Gaelic times, land was measured in terms of its economic potential rather than in fixed units of measurement: by the number of cattle that an area of pasture land could support, or by the time taken to plough an area of arable land. Therefore the size of an "acre" in this system could vary enormously depending on the quality of the land.)
As explained previously, townland size was often determined by the fertility of the land, thus townlands in high quality land tended to be smaller, while townlands in mountainous or bog areas tended to be much larger in size. In many areas of Norman settlement, townland boundaries tend to follow field or individual property boundaries and may reflect the holdings of monasteries or churches or the boundaries of commonage. In these areas, townlands often have apparently irregular boundaries and are of small size. In contrast, townlands in areas of traditional Gaelic settlement tend to be larger in area and usually have apparently regular boundaries determined by streams, rivers or roads.
A useful source of information on townlands (with an emphasis on the northern parts of Ireland) is the Federation for Ulster Local Studies. Its publications include Every Stony Acre Has a Name: Celebration of the Townland in Ulster by Tony Canavan, and Townlands in Ulster: Local History Studies, edited by W.H. Crawford & R.H. Foy.
Townlands in Scotland were often in contradistinction to kirktouns (Clachan), which were settlements with a church, sometimes of ecclesiastical origin.
See also Township (Scotland) for the crofting context.
There may be similarities between the notion of townlands in Ireland and the traditional land divisions of treens (c.f. the Irish word trian, a third part) in the Isle of Man. Treens are subdivided into smaller units called quarterlands
Cornerstone: Fight to Keep Our Townland Names; the Increasing Trend to Diminish Townland Names Has Led Many to Take Action, Hoping That Our Heritage Will Be Preserved. ANNE PALMER Reports
Feb 15, 2002; Byline: ANNE PALMER townland names across Northern Ireland have been used as postal addresses for thousands of years,...