This is a cultural and intellectual history of the Ordnance Survey, which mapped Ireland from 1824 to 1846. Captain Thomas Larcom of the Survey intended to produce and encyclopaedia-like series of county memoirs to accompany the maps, a great survey that would explain Ireland literally, as the maps would represent it graphically. Only one memoir (for Templemore, County Derry), was published before the project was suspended by not before and immense amount of research had been undertaken for the whole country. These memoir reports by Ordnance engineers, scholars and local civic assistants constitute a remarkable archive on culture, folklore, religious practices, oral histories and social structures, before much was swept away by the Famine, modernization and anglicization. This study establishes the critical importance of the Ordnance Survery in nation building.