The Ordnance Survey (OS) was originally established in Britain in 1791, and in 1824 the Survey established its Irish headquarters in Dublin, in order to undertake the mapping and measurement of Irish townlands. The maps were drawn on a scale of 6 inches to one mile and published between 1833 and 1846. They were later revised on a county-by-county basis.

The Ordnance Survey took its name from the Board of Ordnance, a military organisation, and the field surveys were carried out by men of the Royal Engineers who were based in various barracks around the country. In time they became better known as ‘The Sappers’, and they had to determine the exact boundaries of over 60,000 townlands and to record the local spelling of each one of them.

There were four OS centres in Ireland—Dublin, Belfast, Cork, and Ennis. The Ordnance Survey House in College Road, Ennis, now Maoin Cheoil an Chláir, became their local headquarters in Clare, and was the centre of operations for the survey and mapping of Clare, Limerick, part of Galway, and of Kerry, in the earlier stages of the work, and later Sligo, Leitrim and Roscommon. The entire survey was under the direction of Colonel Thomas Colby who realised the need, not just to produce a series of accurate and realistic maps, but also to complement it with a separate topographical inventory of all places of interest in each county, as well as aspects of architecture and archaeology.

George Petrie, the well known antiquarian, was in charge of the topographical department of the OS from 1833 to 1839, evaluating the vast volume of recorded information coming in from the surveying officers field-books. He was ably assisted by John O’Donovan and Eugene Curry, whose work took them into every parish in the country, to identify various field monuments, and to assist in the authentication of local placenames. The results of these investigations are contained in a number of volumes generally referred to as the Ordnance Survey Letters, three of which contain information relative to the antiquities of County Clare, collected during the progress of the Survey in the county in 1839. The OS Field Name Books record townland names in English and Irish, as well as their meanings, in addition to a variety of other information.

The survey was completed in 1846 and led to the publication of 1,906 superbly-drawn engraved maps which illustrated the entire country. The 72 maps devoted to County Clare were published in 1842. The maps were well received in all quarters and fully merited the viewpoint that Ireland had now ‘become the best mapped country in the world.’

Clare County Library holds two complete sets of the 1st edition 6-Inch maps for County Clare. The set scanned for this ‘Maps of County Clare’ project was acquired from the O’Brien family of Dromoland Castle. The second set was once owned by the antiquarian Thomas J. Westropp and used by him in his extensive surveying of the antiquities of the county. They contain some of his corrections and amendments to the original maps and were presented to the Clare Historical Society by Lieutenant-Colonel Conor O’Callaghan Westropp of Maryfort, Lismeehan, Tulla. Both sets of maps are deposited in Clare County Library’s Local Studies Centre in Ennis.