The area of Ireland is divided into 20 squares, measuring 100km by 100km, each identified by a single letter.
Grid Square references are made up of a combination of the easting and northing and consist of a letter and an even number of digits. The letter designates one of the twenty 100 kilometre squares that cover Ireland, as shown by the map above. The remaining numbers represent a point within that 100 kilometre square by giving the distance east and north from the bottom southwest corner of the square. The numbers can vary in numbers of digits depending upon the resolution of the grid reference.
For example: Belfast City Hall has an easting 333831 and northing 374015 which means it is 333831 metres east and 374015 metres north of the bottom southwest corner. This puts it in 100km square J. The distance from the southwest corner of square J is 33 831 metres east and 74 015 metres north. The grid reference of the City Hall is thus J 33 74 to a 1km resolution and J 338 740 to a 100m resolution.
Census Office Grid Squares
The grid-squares used for Census purposes uses this grid reference to refer to the area enclosed by a square projected from that grid reference by the distance that the resolution of the grid reference dictates. Thus a grid-square with reference J 33 74 extends 1km east and 1km north from that point while the grid-square with reference J 338 740 extends 100m east and 100m north.
Easting and Northing Point References
A grid reference for a point location is made up of a combination of the easting and northing from this location.
For example: Belfast City Hall has an easting 333831 and northing 374015 which means it is 333831 metres east and 374015 metres north of the bottom southwest corner. The Giants Causeway has an easting 294724 and northing 444769 which means it is 294724 metres east and 444769 metres north of the bottom south west corner. Finally Enniskillen Castle has an easting 223141 and a northing 344221.