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".
Under British rule, the Irish mile was not always used. From 1774 until the 1820s, the grand juries of 25 Irish counties commissioned maps at scales of one or two inches per Irish mile but the County Mayo maps (1809–1830) were surveyed and drawn by William Bald in English miles and just rescaled to Irish miles for printing. The Ordnance Survey of Ireland, from its establishment in 1824, used English miles. Thomas Telford's Howth–Dublin Post Office extension of the London–Holyhead turnpike had its mileposts in English miles. Irish measure was formally abolished by the 1824 Weights and Measures Act but the Irish Post Office continued to use the measure until 1856.
Several post-1824 statutes used Irish miles. One was the Lighting of Towns (Ireland) Act, 1828, which allowed those residing within one Irish mile of a town market to vote on whether to establish town commissioners. Another was the Parliamentary Boundaries (Ireland) Act, 1832 (passed in tandem with the Irish Reform Act 1832) which defined the radius of the parliamentary borough of Sligo as "One Mile, Irish Admeasurement, from ... the Market Cross"; the same as the boundary established for local taxation purposes in 1803.
Prior to the publication of standardised traffic regulations by the Irish Free State in 1926, signage varied from county to county, prompting complaints from travellers such as Alfred Austin. In 1902, the Royal Royal Book of Ireland explained that "Counties Dublin, Waterford, Cork, Antrim, Down, and Armagh use English, but Donegal Irish Miles; the other counties either have both, or only one or two roads have Irish". The 1909 "Thorough" Guide said, "The Railway Companies adopt English miles. The [horse] car proprietors are apt to be elastic in their choice. The Counties of Dublin, Waterford, Cork, Antrim, and Armagh use English milestones, Donegal uses Irish only, and the other counties either have both or a mixture. Metal milestones, however, show English, and stone ones Irish, miles." The Oxford English Dictionary’s 1906 definition of "mile" described the Irish mile as "still in rustic use".
The Irish Free State standardised its roads using English statute miles, leading to some nationalist complaints. In 1937, a man being prosecuted for driving outside the 15-mile limit of his licence offered the unsuccessful defence that, since Ireland was independent, the limit should be reckoned by Irish miles "just as no one would ever think of selling land other than as Irish acres". In 1965, two deputies proposed an amendment to the Road Transport Act to replace the English statute miles with Irish ones; it was rejected. Such complaints—and the traditional distance itself—are now considered obsolete following Irish metrication from the 1970s; however, "an Irish mile" is still used colloquially to express a vague but long distance akin to a "country mile".
Two-Mile Borris, County Tipperary is two Irish miles from Leighmore, site of a medieval monastery. Threemilehouse, County Monaghan is three Irish miles from Monaghan town. Fivemiletown, County Tyrone is five Irish miles equidistant from Clogher, Brookeborough and Tempo. Sixmilebridge, County Clare is six Irish miles from Thomondgate, Limerick. Sixmilecross, County Tyrone is six Irish miles from Omagh. The name of Six Mile Water, County Antrim is said to derive from the crossing point six Irish miles from Antrim town on the road to Carrickfergus.
The Irish acre or plantation acre measured one Irish chain by one Irish furlong, or 4 Irish perches by 40, or 7840 square yards: approximately 0.66 hectares or 1.62 statute acres. The Lancashire acre around the Solway Firth and the Churchland acre in Yorkshire were the same size, which Frederic Seebohm in 1914 connected to the erw of Gwent in Wales. A 1653 survey of lands of the Countess of Ormonde in County Kilkenny used statute acres, whereas the 1654–5 Civil Survey and 1655–6 Down Survey used plantation acres. Irish acres were used in the 1823–37 applotments made under the Composition for Tithes (Ireland) Act 1823. The Ordnance Survey of Ireland from its 1824 foundation used statute acres in its maps, which were used in turn for Griffith's Valuation and the census. The Irish acre remained common in Irish newspaper advertisements for farmland and other property until the middle of the 20th century.
In older Gaelic usage, a townland was notionally 60 or 120 "acres", but the size varied by the quality of the land. This unevenness was not sufficiently understood by English and Scottish planters, which caused disputes when confiscated land was divided and assigned. In Anglo-Norman Ireland, as in England of the time, "acre" sometimes meant any individual plot of land; as a standard measure, it was probably about 2.5 statute acres near Dublin, perhaps differing elsewhere. Of eleven townlands in Ireland named "Fortyacres" in 1901, six had areas between 60 and 78 statute acres; the smallest was 46 acres and the largest 185. The civil parish of Carn, County Wexford has townlands named Threeacres, Nineacres, and Nineteenacres, with respective areas of 12, 21, and 30 statute acres.
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