Old Irish units of measurement


texts record a wide variety of units of measurement, organised into various systems. These were used from Early Christian Ireland or perhaps earlier, before being displaced by Irish measure from the 16th century onward.

Length

A troighid was the length of a man's foot, divided into twelve ordlach, "thumb-lengths". These figures assume a man's foot to measure 10 in.
A magh-space was a unit set at the distance from which a cock-crow or bell could be heard. Other units such as inntrit and lait appear in documents; their value is uncertain, perhaps being equivalent to 1 and 2 fertachs respectively.
Ancient Laws of Ireland reads ceithri orlaighi i mbais, teora basa i troighid. and
"Catalogue of the Irish manuscripts in the British Museum v.1" gives ceithri gráine an t-órdlach.
Stair Ercuil ocus a bás: the life and death of Hercules mentions ceim curadh.

Area

The basic unit of area was the tir-cumaile, "land of three cows", as it was an area of land that was at some point worth three cows. It is sometimes erroneously interpreted as the area needed to graze three cows, but it is far too large for that; in modern Ireland, a cow grazes on about 0.4 ha, so twenty or more could graze a tir-cumaile. Ireland in total covered about 870,000 tir-cumaile.

Capacity

A hen's eggshell was used as a standard unit, roughly 55 ml.

Mass

lists two types of unge: unge mór at 20 pennyweights and unge beg at 10 pennyweights.
A screpall óir was used for measuring gold weight and was equal to a quarter-ounce.

Time

A night was used as a measure for time in preference to a day. Unlike most traditional societies, the Irish held that a new day began at sunset, not at sunrise, so that a Wednesday night would precede the day of Wednesday.