Irish acre

Irish measure or plantation measure was a system of units of land measurement used in Ireland from the 16th century plantations until the 19th century, with residual use into the 20th century. The units were based on "English measure" but used a linear perch measuring 7 yards (6.4 m) as opposed to the English rod of 5.5 yards (5.0 m). Thus, linear units such as the furlong and mile, which were defined in terms of perches, were longer by a factor of 14:11 (~27% more) in Irish measure, while areas such as the rood or acre were larger by 196:121 (~62% more). After the Act of Union 1800, Ireland was part of the United Kingdom of Great Britain and Ireland, whose Parliament passed the Weights and Measures Act 1824, which established English measure in Ireland as "Imperial measure" or "statute measure". Imperial measure soon replaced Irish measure in the use of the Dublin Castle administration, but Irish measure persisted in local government, and longer still in private use. A third system, "Scotch measure" or "Cunningham measure", was also used in Ulster Scots areas.

The Irish mile (míle or míle Gaelach) as latterly defined measured exactly 8 Irish furlongs, 320 Irish perches, or 2240 yards: approximately 1.27 statute miles or 2.048 kilometres. During the Elizabethan era, 4 Irish miles were generally equated to 5 English ones although whether this meant the old English mile or the shorter statute mile is unclear. The 21-foot perch was in use by the time of William Petty. However, a 1715 statute of the Parliament of Ireland defines the fare for ferries in terms of "common Irish miles: (that is to say) at one English mile and an half or twelve furlongs at least to each mile".

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