Old Irish units of measurement - AbsoluteAstronomy.com

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 25 cm (10 inches).

Table of length units
Unit	Meaning	Relative value	SI value	Imperial value	Notes
grain	grain	1/36	0.7 cm	1/4 inch
ordlach	thumb-length	1/12	2.1 cm	0.8 in
bas	palm	1/3	8.4 cm	3.3 in
dorn	fist	5/12 or 1/2	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	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.

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 Si Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system... 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 Si Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system... value	Imperial value	Notes
	eggshell	1	55 ml	2 fl. oz.
méisrin		12	660 ml	1 pint 3 fl. oz.

Mass

Table of mass units
Unit	Meaning	Relative value	SI Si Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system... value	Imperial value	Notes
	grain	1/8	0.05 g	0.772 Gr	One grain of the best wheat Wheat Wheat is a cereal grain, originally from the Levant region of the Near East, but now cultivated worldwide. In 2007 world production of wheat was 607 million tons, making it the third most-produced cereal after maize and rice... .
pinginn	penny	1	0.4 g	6.173 Gr
screpall	scruple	3	1.2 g	18.519 Gr
unga	ounce	72	28 g	1 oz	Word borrowed from the Latin Latin Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and... uncia, although an older native name was mann.

Time

A night

Night

Night or nighttime is the period of time when the sun is below the horizon. This occurs after dusk. The opposite of night is day...

(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.

Sources