The Down Survey, so called because a chain was laid down and a scale made, was taken from 1656-8 under the direction of William Petty. Using the Civil Survey as a guide, teams of surveyors, mainly former soldiers, were sent out under Petty’s direction to measure every townland to be forfeited to soldiers and adventurers. The resulting maps, made at a scale of 40 perches to one inch (the modern equivalent of 1:50,000), were the first systematic mapping of a large area on such a scale attempted anywhere. The primary purpose of these maps was to record the boundaries of each townland and to calculate their areas with great precision. The maps are also rich in other detail showing churches, roads, rivers, castles, houses and fortifications. Most towns are represented pictorially and the cartouches, the decorative titles, of each map in many cases reflect a specific characteristic of each barony.
A more detailed history is available at http://downsurvey.tcd.ie/history.html.
While the surviving original maps were destroyed in the Custom House Fire in 1922, many copies of parish maps were stored elsewhere. Similar maps made by Petty and his team at the same time as the Down Survey was being compiled also were not in the Custom House at the time of the fire. One set of maps, the Hibernia Regnum, was captured by French privateers in 1707 from the vessel Unity, while en route from Dublin to London. The maps eventually were given to the French Imperial Library. These maps have been used to create a new web site that displays the available maps.
These maps and a lot more information have now been placed online on a web site created by Trinity College Dublin. If you can trace your ancestors back to the 1600s, you may be able to find a lot more about them by using the maps of the Down Survey of Ireland web site. Not only will you find names, but eve maps of their villages, showing roads, churches, and even buildings.
The maps chart the changes in land ownership in Ireland. Users can search by the names of landowners in 1641 and in 1670. It also displays ownership by religion. The web site also displays a lot of other historical information, such as roads and even a Map of the Ulster 1641 Depositions showing the number of recorded murders in each townland.
All this and a lot more is available at no charge at http://downsurvey.tcd.ie/.
My thanks to Richard Heaton for telling me about this rich source of information.