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Ordnance Survey Ireland (OSi; Irish: Suirbhéireacht Ordanáis Éireann) is the mapping agency in the Republic of Ireland and together with the Ordnance Survey of Northern Ireland, succeeded, post-1922, the Irish operations of the United Kingdom Ordnance Survey.

The Irish Survey was established in 1824, along similar lines to the Ordnance Survey in Great Britain, to provide a highly detailed (1:10560, 6 inches to 1 mile) survey of the whole of the island of Ireland, a key element in the process of equalising local taxation.

From 1825-46, teams of surveyors, led by officers of the Royal Engineers and men from the ranks of the Royal Sappers and Miners, traversed Ireland, reating a unique record of a landscape undergoing rapid transformation. The resulting beautiful maps (primarily at 6" scale, with greater detail for urban areas, to an extreme extent in Dublin) portrayed the country in a degree of detail never attempted before, and when the survey of the whole country was completed in 1846, it was a world-first. Both the maps and surveying were executed to a very high degree of engineering excellence, using triangulation and with the help of tools developed for the project, most notably the strong "limelight."

The Engineer officers in charge of the operation were Lt-Colonel Thomas Colby, a long-serving Director-General of the Ordnance Survey, and Lieut Thomas Larcom. They were assisted by George Petrie, who headed up the Survey's Topographical Department which employed the likes of John O'Donovan and Eugene O'Curry in scholarly research into place-names.

This "mapping scheme" provided numerous opportunities for employment to the native Irish people, both as skilled/semi-skilled fieldwork labourers and as clerks in the subsidiary Memoir project that was designed both to illustrate and complement the maps by providing hard data on the social and productive worth of the country.

The total cost of the Irish Survey was £860,000.

The original survey was later revisited and revised maps issued on a number of occasions. All of these historical maps (at least up to 1922) are in the public domain and while originals can be hard to locate, can be freely reproduced.

In more recent times, the Ordnance Survey of Ireland replaced traditional ground surveying with mapping based primarily on aerial photography. It has also worked with An Post to gather and structure geographic data.

OSi is now an autonomous agency, with a remit to cover its costs of operation from its sales of data and derived products, which has, as with the Ordnance Survey in the UK, sometimes raised concerns about the mixing of public responsibilities with commercial imperatives.

• Irish national grid reference system, the reference grid used in both the Republic of Ireland and Northern Ireland.

Andrews, J.H., A Paper landscape: the Ordnance Survey in nineteenth-century Ireland (Oxford, 1975).

McWilliams, P.S., "The Ordnance Survey Memoir of Ireland: Origins, Progress and Decline" (PhD thesis, Queen's University Belfast, 2004).

Report on Ordnance Memoir (1843), HC 1844 (527) xxx, 259-385.

• Official website of OSi
• Official website of OSNI
• Online map browser showing Irish Townland Maps Surveyed between 1829 and 1843 - covers the whole of Ireland
• Online Ordnance Survey Maps of County Clare
• Online Ordnance Survey Maps of County Mayo
• Online Ordnance Survey Maps of County Galway
• Online Ordnance Survey Maps of County Sligo