Old Irish units of measurement

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

Length

A troighid ("foot") 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 (25 cm).

Table of length units
Unit Meaning Relative
value
SI
value
Imperial
value
Notes
grán grain 136 0.7 cm 1/4 inch
ordlach thumb-length 112 2.1 cm 0.8 in
bas palm 13 8.4 cm 3.3 in
dorn fist 512 or ​12 10.4 or 12.5 cm 4 or 5 in
troighid foot 1 25 cm 9.9 in
céim step 2.5 62.5 cm 2 ft 1 in
deiscéim[m] double-step 6 1.5 m 4 ft 11 in
fertach rod 12 3 m 9 ft 10 in
forrach 144 36 m 39.4 yards

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 (4 thumb-lengths in a palm, 3 palms in a foot).[1] and

"Catalogue of the Irish manuscripts in the British Museum v.1" gives ceithri gráine an t-órdlach (4 grains in the thumb-length).

Stair Ercuil ocus a bás: the life and death of Hercules mentions ceim curadh (warrior's paces).

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.

Table of area units
Unit Meaning Relative
value
SI
value
Imperial
value
Notes
tir-cumaile land of three cows n/a 9.3 ha 23 ac 72 square forraigh
achar acre 1 Adopted following Norman invasions.
seisrech plough-land 120
baile townland 1,440 The term "townland" is still used to designate very small named areas in the countryside.
tuath 4,320 Originally a term for a petty kingdom.

Capacity

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

Table of capacity units
Unit Meaning Relative
value
SI
value
Imperial
value
Notes
og eggshell 112 55 ml 2 fl. oz.
méisrin little measure 1 660 ml 1 pint 3 fl. oz.
olderb great vessel 12 7.92 L 1.74 gallons
oilmedach great mead [vessel] 144 95 L 21 gallons

Mass

Table of mass units
Unit Meaning Relative
value
SI
value
Imperial
value
Notes
grán grain 18 0.05 g 0.772 Gr One grain of the best wheat.
pinginn, penginn pennyweight 1 0.4 g 6.173 Gr
screpul(l), scripul(l), screaball scruple 3 1.2 g 18.519 Gr
ungae, uinge ounce 72 28 g 1 oz Word borrowed from the Latin uncia, although an older native name was mann.

The Manners and Customs of Ireland lists two types of unge: unge mór at 20 pennyweights (31.1 g) and unge beg at 10 pennyweights (15.6 g).[2]

A screpall óir (gold scruple) was used for measuring gold weight and was equal to a quarter-ounce (7 grams).[3]

Time

A night (oídhche) 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.

References

• Joyce, P. W. (1906). "23: Measures, Weights, Mediums of Exchange". A Smaller Social History of Ancient Ireland. Retrieved 20 May 2015.
• "tir cumaile [Old Irish]". sizes.com. Sizes, Inc. 1 September 2008. Retrieved 20 May 2015.
• Seebohm, Frederic; Hugh Exton, Seebohm (1914). "2: The Irish units of tribute and food-rent". Customary acres and their historical importance, being a series of unfinished essays". Longman's, Green, & co. Retrieved 20 May 2015.