Irish Grid (also referred to as IG and Irish National Grid) was created to allow Ireland, as it sits on the curved surface of our spherical planet Earth, to be represented more accurately on flat maps. As a result of angularly spaced lines of longitude used in the global coordinate system getting closer together as they approach the True North Pole, 1° of east-west at Mizen Head in the south of Ireland measures 69.4km, while the same at Malin Head in the north only measures 63.2km. When expanded across the full 5° width of Ireland, that 6.2km deviation per degree scales up to 31km.
Irish Grid is the local coordinate system covering the whole of Ireland. Based on metric units of length, Irish Grid appears on modern maps as two sets of equally-spaced parallel lines - one set running left to right and the other running perpendicularly up and down - to form a grid of squares. Reference numbers for the grid lines appear within each map and around its frame.
Maps are produced as rectangular sheets because this is the most cost effective and efficient format. It therefore makes sense for their coordinate system to be based on parallel horizontal (east-west) lines and parallel vertical (north-south) lines, like those used in Irish Grid.
Please note that Ordnance Survey Ireland (OSi) and the Ordnance Survey of Northern Ireland (OSNI) introduced a new coordinate system for Ireland in 2001 called Irish Transverse Mercator (ITM). With its 1km x 1km square grid, ITM is very similar to Irish Grid. The most noticeable difference being that its origin is roughly 400km west and 500km south of Irish Grid's. The reason for introducing this new system was to make it easier for GPS equipment to convert raw data from the global Universal Transverse Mercator (UTM) coordinate system. It was originally intended to replace Irish Grid with ITM. However, as of 2025, Irish Grid continues to be the primary coordinate system appearing on printed consumer maps used by recreational walkers. Irish Grid is the system used in the online OSNI's Spatial NI map viewer. It is also an option on OSi's GeoHive map viewer, where ITM is the default. ITM is primarily used today by professional land surveyors in Ireland. Even so, surveyors generally provide their final product to clients in Irish Grid format so that it can be used with OSi and OSNI digital vector mapping. For the foreseeable future, recreational walkers and other outdoor enthusiasts are not required to abandon the trusted Irish Grid.
Irish Grid is the local coordinate system covering the whole of Ireland. Based on metric units of length, Irish Grid appears on modern maps as two sets of equally-spaced parallel lines - one set running left to right and the other running perpendicularly up and down - to form a grid of squares. Reference numbers for the grid lines appear within each map and around its frame.
Maps are produced as rectangular sheets because this is the most cost effective and efficient format. It therefore makes sense for their coordinate system to be based on parallel horizontal (east-west) lines and parallel vertical (north-south) lines, like those used in Irish Grid.
Please note that Ordnance Survey Ireland (OSi) and the Ordnance Survey of Northern Ireland (OSNI) introduced a new coordinate system for Ireland in 2001 called Irish Transverse Mercator (ITM). With its 1km x 1km square grid, ITM is very similar to Irish Grid. The most noticeable difference being that its origin is roughly 400km west and 500km south of Irish Grid's. The reason for introducing this new system was to make it easier for GPS equipment to convert raw data from the global Universal Transverse Mercator (UTM) coordinate system. It was originally intended to replace Irish Grid with ITM. However, as of 2025, Irish Grid continues to be the primary coordinate system appearing on printed consumer maps used by recreational walkers. Irish Grid is the system used in the online OSNI's Spatial NI map viewer. It is also an option on OSi's GeoHive map viewer, where ITM is the default. ITM is primarily used today by professional land surveyors in Ireland. Even so, surveyors generally provide their final product to clients in Irish Grid format so that it can be used with OSi and OSNI digital vector mapping. For the foreseeable future, recreational walkers and other outdoor enthusiasts are not required to abandon the trusted Irish Grid.
Examples of Irish Grid on maps
The slideshow below shows examples of how Irish Grid appears on 1:250,000, 1:50,000 and 1:25,000 mapping.
The 100km Irish Grid squares
Irish Grid covers a square portion of the Earth's surface measuring 500km x 500km. Its origin (0,0) at its southwest corner is located off the coast of Counties Cork and Kerry. The grid is split into twenty-five 100km x 100km squares lettered from A in the northwest to Z in the southeast (excluding I). Ireland is covered in its entirety by eighteen of these squares: they are B, C, D, F, G, H, J, L, M, N, O, Q, R, S, T, V, W and X. Grid squares A, E, K, P, U, Y and Z are outside Ireland's land area. It is unclear why the eastern column of squares (E, K, P, U and Z) were included in the Irish Grid system, although it may be related to marine navigation in the Irish Sea, North Channel and St George's Channel. Squares H and N are the only ones without any sea area.
Grid square A is the only one containing no land at all. Parts of southwest Scotland are covered by squares C, D, E, J and K; the western edge of Cumbria by E and K, the entire Isle of Man by K; west Wales by P, U, Y and Z; and a small portion of Devon and Somerset by Z. Maps for all those other land areas use British Grid which interestingly overlaps Irish Grid to such an extent that it also covers the eastern half of Ireland.
You can see how Irish Grid's twenty-five lettered 100km x 100km squares (black lines) relate to local lines of longitude (blue lines) in the map below. Please note that the distorted projection used by Google Maps displays lines of longitude running parallel to each other. For this reason, Irish Grid appears to widen northward. The image below the map shows a more accurate Google Earth representation of this relationship where the lines of longitude converge toward the True North Pole.
The image below shows the same relationship between the two coordinate systems as they appear in Google Earth where Ireland's shape matches more closely to its actual convex shape on the globe. For this reason, Irish Grid's twenty-five lettered 100km x 100km squares (black lines) are less distorted and the lines of longitude (white lines) can be seen converging quite noticeably towards the True North Pole.
In both the Google Map and the Google Earth image above, the red vertical line highlights longitude 8° West which conveniently marks roughly the central north-south axis of Ireland. As such, this particular line of longitude was chosen as the setting out baseline for Irish Grid where it now matches the 200km easting line.
The north-south positioning of Irish Grid appears to have been based on the full or half degree latitude line that is closest to midway between Ireland's southernmost and northernmost latitudes (i.e. 51.39° North and 55.45° North respectively). Based on this, the closest latitude is 53.5° North, which is just under 0.1° north of actual midway. From this, the setting out basepoint for Irish Grid was identified as 53.5° N, 8° W which translates as 200000,250000 or N 00000 50000 in Irish Grid (marked as a red circle in the Google Map above). This point is located in Lough Ree approximately 110 metres from its southwest shore near Curry Hill at Carnagh Military Range in County Roscommon.
Using the 8° West baseline and 53.5° N, 8° W basepoint, surveyors and cartographers were then able to accurately set out the full grid of twenty-five lettered 100km grid squares.
The 1km Irish Grid squares
Each lettered 100km x 100km Irish Grid square is further split into its own grid of 1km x 1km squares (10,000 per lettered square). These appear on the medium scale maps used by recreational walkers, enabling us to work out coordinates fairly accurately by eye to within 50 metres. With the aid of a suitable measuring device (e.g. a romer on a compass), it is possible to achieve an even higher degree of accuracy.
The image below shows an example of a 1km x 1km Irish Grid square as it appears on a 1:25,000 map. At this scale, the square measures 4cm x 4cm on the printed map, where every millimetre represents 25m. On a 1:50,000 map, the square would be 2cm x 2cm, with each millimetre representing 50m.
True North / Grid North
True North is the direction you would travel to arrive at the True North Pole. This location in the Arctic Ocean marks one end of the axis around which planet Earth spins once every 24 hours. All lines of longitude converge to pass through this location, therefore each of these lines points toward True North (and True South, in the opposite direction). True North is mainly used with the Latitude / Longitude coordinate system. As such, it is rarely used by the Irish hillwalking community, if ever.
Grid North relates to a local coordinate system (e.g. Irish Grid) where a set of equally-spaced parallel north-south lines and a set of equally-spaced parallel east-west lines form a grid of uniform squares. In this type of coordinate system, each of the parallel north-south lines points toward Grid North. One of these north-south grid lines is likely to point toward True North where it matches a line of Longitude (the setting out baseline). In Irish Grid, the setting out baseline is 8° West.
On longitude 8° West, which is also Irish Grid's 200km easting line, True North and Grid North are exactly the same (i.e. toward the True North Pole). The further west of longitude 8° West you travel, Grid North deviates more and more to the west of True North. The further east of longitude 8° West you travel, Grid North deviates more and more to the east. For each individual location in Ireland, the angle between its Grid North and its True North never changes. From a hillwalking perspective, this angle rarely comes into play, because True North is really only used with the Latitude / Longitude coordinate system. The angle you really need to know about as a skilled navigator is the constantly changing one between Grid North and Magnetic North.
In the diagrams below, GN is Grid North, TN is True North, and MN is Magnetic North as they appear on maps based on the Irish Grid coordinate system depending on which part of Ireland a specific map covers. As you can see, Grid North always points straight up the map. Please note that the angles between the various Norths are purely indicative - and exaggerated - in the diagrams below, except where it states that GN=TN at longitude 8° West.
Magnetic North - the only North that can move - is steadily creeping across the Arctic Ocean from Canada towards Siberia. It is expected that Magnetic North's movement during the 2030s will require the red arrow positions to be updated in the diagrams below if it continues to travel on its current trajectory. Magnetic North is not related to any coordinate system, but is worthy of a mention here.
Please note that the set of diagrams below was created for the purpose of illustrating the current relationship between the three Norths in Ireland only.
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