The OpenPostcode is an opensource proposal for a compressed geolocation postcode. Settings for Ireland are scaled to an Irish GPS Box, including the whole island from 55.5N 10.8W to 51.3N 5.4W. Its format can be described as any number of characters from an alphanumeric alphabet. Each character encoding a longitude and latitude in a base calculated from the integer component of the square-root of the number of available characters. For Ireland the alphabet is "6789B5NPQC4MXRD3LWTF2KJHG" (which avoids accidental words by removing vowels and "Y", the numbers 1 and 0 and letter "V" which can be confused with vowels, and the letters "S" and "Z" which can easily be confused with numbers - 25 characters of which each character is both a longitude and latitude encoding of base 5 (both numbers can happily coexist in a single character) and so describes a grid of 5 by 5 about the country in one character with each additional character added subdividing by a further 25. For Ireland, in particular, the grid is described as a spiral of its 25 characters starting
with 2 in the bottom left and continuing clockwise to X in the middle. Thus X
will always be the centre, and all codes beginning with "B" are firmly to-right, within the UK postcode area
BT, to avoid any possible confusion. At eight characters the code in Ireland defines an area well inside a square meter throughout the country. The code is by design extensible and describes an ever increasing precision around a position with each additional character - open to a variety of independent applications and formats. After a defined separator (a forward slash), an optional checksum character from an alphabet of suitable characters ("0123456789ACDEFGHJKLMNPQRTUVWXY" giving 31 characters, avoiding "B", "I", "O", "S", and "Z") is calculated as, e.g. modulus 31 of the sum of an ordered weighting (leftmost multiplied by 1, next by 2, etc.) of character values and the integer of the parameters west*north-wide*high (which amounts to zero in the Irish calculation and so can be ignored).
(OPC Version 4, June 2015)
Some examples in the documentation here are still using version 3 so are incompatible with the updated demo map and app. The algorithm is the same but uses the alphabet "6789B5NPQC4MXRD3LWTF2KJHG".