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.
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.
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.
UCD's Historic OS Maps USER GUIDE helps you navigate these resources (includings how to find an old OS map's survey date).