The national grid referencing system was devised by the Ordnance Survey, and is heavily used in their survey data, and in maps (whether published by the Ordnance Survey of Ireland, the Ordnance Survey of Northern Ireland or commercial map producers) based on those surveys. Additionally grid references are commonly quoted in other publications and data sources, such as guide books or government planning documents.
The Ordnance Survey of Ireland has implemented a new coordinate system for Ireland called Irish Transverse Mercator, or ITM, which will initially run in parallel with the existing Irish grid system. In both systems, the true origin is at 53° 30' N, 8° W — a point in Lough Ree, close to the western (Co. Roscommon) shore, whose grid reference is N 000 500.
The area of Ireland is divided into 25 squares, measuring 100km by 100km, each identified by a single letter. The squares are numbered A to Z with I being omitted. 7 of the squares do not actually cover any land in Ireland - A, E, K, P, U, Y and Z.
Within each square, eastings and northings from the origin (south west corner) of the square are given numerically. For example, G0305 means 'square G, 3 km east, 5 km north'. A location can be indicated to varying resolutions numerically, usually from two digits in each coordinate (for a 1 km square) through to five (for a 1 m square); the most common usage is the six figure grid reference, employing three digits in each coordinate to determine a 100 m square.
Coordinates may also be given relative to the origin of the entire 500 by 500km grid (in the format easting, northing). For example the location of the Spire of Dublin on O'Connell Street may be given as 315904, 234671 as well as O1590434671. Coordinates in this format must never be truncated, because, for example, 31590, 23467 is also a valid location.
The Irish national grid reference system is the system of grid references used in Northern Ireland and the Republic of Ireland. It overlaps the British national grid reference system. In the grid references, Ireland is divided into 20 squares, of 100 kilometres length and 100 kilometres width. Each grid square is given a letter as its name.