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.
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 25cm (10inches).
| Table of length units | ||||||
|---|---|---|---|---|---|---|
| Unit | Meaning | Relative value | SI value | Imperial value | Notes | |
| grán | grain | 0.7 cm | 1/4 inch | |||
| ordlach | thumb-length | 2.1 cm | 0.8 in | |||
| bas | palm | 8.4 cm | 3.3 in | |||
| dorn | fist | or | 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).
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. | ||||
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 | 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 | ||
| Table of mass units | ||||||
|---|---|---|---|---|---|---|
| Unit | Meaning | Relative value | SI value | Imperial value | Notes | |
| grán | grain | 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 (gold scruple) was used for measuring gold weight and was equal to a quarter-ounce (7 g).[3]
A night (oídhche) was used as a measure for time in preference to a day. As was normal for Islam and Jews and in line with the Bible (It was evening and morning of the first day), the Irish held that a new day began at sunset, not at sunrise, so that a Wednesday night would precede the day of Wednesday.