The Ordnance Survey Office, established in 1824, was created to carry out a military survey of the entire island of Ireland to update land valuations for land taxation purposes. The original survey at a scale of 6 inches to 1 mile was completed under the direction of Major General Colby. Ireland thus became the first country in the world to be entirely mapped at such a detailed scale. When the survey of the whole country was completed in 1846, it was a world first.
Major Thomas Frederick Colby (Director of the First OS)
Particularly relevant for genealogy or those with an interest in the history of a place, these "6-inch" maps include every tiny house or cabin that was likely occupied before the Famine. The good news is that the OSI has captured this and later mapped data in a digitised format, and they are free to inspect online.
The first edition of these maps (the 6-ich colour maps) and additional mapping data can be explored free online via OSi (Ordnance Survey Ireland) and OSNI (Ordnance Survey of Northern Ireland).
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OSI's GeoHive MAP VIEWER provides free, web-based access to Ordnance Survey Ireland and other geospatial datasets. For OS 6-inch and 25-inch County Series historical maps see its National Townland and Historical Map Viewer.
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OSNI's Spatial NI MAP VIEWER aka PRONI Historical Maps Viewer allows access to Ordnance Survey maps and other geospatial information for Co. Antrim, Co. Armagh, Co. Down, Co. Derry ~ Londonderry, Co. Fermanagh, and Co. Tyrone.
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UCD's Historic OS Maps USER GUIDE helps you navigate these resources (includings how to find an old OS map's survey date).
READ MORE A short history of the Ordnance Survey on Ireland ALSO Finding & Navigating Old Irish Maps
