"The townland is the smallest and most ancient of Irish land divisions, and is the goal of all family researchers in identifying the origin of their ancestors. The townland was named at an early period and they usually referred to a very identifiable landmark in the local area such as a mountain, a bog, an oak forest, a village, a fort or a chruch. The townland became standardised as a basic divison in the 17th century surveys by people with little knowledge of the Irish language. As a consequence many place names were either lost or had their meaning or construction altered. The townland spellings are those used by the Ordnance Survey from the 1830's onwards. This means the townland spelling in records collected before this time, such as the tithe books, are very varied.
To 1898 the civil parish was the major administrative division. The two great surveys of the 19th century - the Tithe assessment and the Griffiths valuation - were compiled on a civil parish basis with head of household only listed by thier townland "address". (unless it was a larger Town, rather than the smaller Townland,there were no streets or street numbers in what you would think of as an 'address')" Inner City Trust 1989

A word about Towns vs Townlands -

"One of the confusing things about Irish government is that the "towns" for the most part really had no legal status. That is, no government function. They were just built-up areas. Therefore, you can't really look for "Ballylongford Town" on the Griffiths Valuation and find anything. It might be as many as a half dozen townlands which contain the areas which comprise the "Town of Ballylongford" or Castleisland or whatever.
Note: After 1898 the principle administrative division for Irish records became the Poor Law Union/ Registration Districts." Ray Marshall