With St Patrick’s day almost upon us, I thought it would be the perfect time to take a look at the traditional Gaelic units of measure that were once commonly used in Ireland.

We have evidence of the units of measure people used in Ireland going back as far as early Christian times (around 400 AD). The measurements that evolved around then lasted until around the 16th century, when measures more closely aligned to English units were introduced during the plantations.

Many of the measures that evolved in Ireland bear a strong resemblance to those of other countries, despite being developed independently. Unsurprisingly, these units were based on parts of the human body — such as the foot-sized troighid, and the thumb-sized ordlach — and with other objects and sizes relatable to people in their everyday lives.

Way back when…

The following are examples of the earliest known Gaelic units, used around the early Christian era.



The smallest unit of length was the grán (pronounced like “grawn”). This was based on the length of a grain of wheat and was just over 14 of a modern inch. This is a similar unit to the barleycorn (13 inch).


The next sizes up are the hand based units, such as the thumb-sized ordlach (or orlach in modern Irish, pronounced like “or-lock”), which was 3 grán — or almost 78 of a modern inch.
The bas (pronounced like “b-aww-ss”) was a palm-sized unit equal to 4 ordlach (about 3 13 modern inches), slightly larger than a palm in English units.
There was also a unit based on the size of a fist, the dorn, which was 6 ordlach (or 5 modern inches).


As you might expect, there were also foot and pace-based units. The old Irish version of the foot was the troighid (or troigh in modern Irish, pronounced like “trig”). As with the English foot, this was 12 ordlach in size, or approximately 10 modern inches. There was also a céim (or step in English, pronounced like “came”) of 2 12 troighid and a double-step deiscéim, which is actually 6 troighid, not 5, and works out at almost 5 feet in modern units.

Longer units of length include the fertach, or rod in English, of 12 troighid. This makes it 9’10” in modern terms — much shorter than the English rod. 12 fertach make a forrach — or just under 40 yards in modern terms.

There doesn’t seem to be much recorded in terms of a similar unit to the Roman mile around this time, but there is one vague unit called a Senchus Mór, which was understood to be “as far as the sound of the bell or the crow of the barn-door cock could be heard”. There was also a faithche, which was the length a man could achieve using a spear while standing at his house. This was the distance away from the house that the boundary.



The original Irish unit of area was the tir-cumaile. A cumal is a term for three cows, and this unit is based on the land required to graze a cumal (tir means land). It was 12 forrach in length and 6 forrach wide, or about 23 acres.

After the Anglo-Norman invasion, and the acre was introduced, some other units of area were common in Ireland, including a seisrech, or “ploughland” of 120 acres. This was equivalent to the English Carucate.

There was also a baile of 12 seisrech/ploughlands (1,440 ac). In English, this translates to a townland, which is still a common term used today in Ireland to refer to areas near a town. Baile or bally is also a common prefix for many towns in Ireland.

30 bailes made a tuath (4,320 ac), which was a word for a petty kingdom or territory.



The ancient Irish used a unit of volume based on the capacity of a vessel that was (and is) commonly available and, for the time, quite convenient to use — a hen’s eggshell. It was called an og (pronounced “oh-g”, the old Irish word for egg), and was approximately 2 fl oz.


12 og made a méisrin (pronounced like “may-sh-rin”), the Irish version of the pint (1 pt 3 fl oz in imperial). 12 méisrin made an olderb (or “great vessel” which equates to 1 34 imp gallons). 12 olderb made an oilmedach (or “great mead vessel”, approximately 21 imp gallons).


The smallest weight was the grán, based on the same thing as the unit of length — a grain of wheat. It was a similar size to the grain in English units and was approximately 0.05 grams.

The pinginn (pronounced “pin-yeen”) was 8 grán and was the Irish version of the pennyweight. This was also the name for the penny in the Irish currency before the switch-over to the Euro. 3 pinginní made a screpall, the Irish word for the scruple, and was about the same size. 24 screpall made an ungae, Irish for ounce (same Latin origins).

More modern times…

After British occupation, and the plantations of the 17th century, English units began to displace the traditional Gaelic measures. However, there were still differences with the Irish versions of these measures until “statute” imperial measures were later enforced. Here are some notable differences:


The Irish mile (or míle Gaelach) was 8 Irish furlongs of 280 yards, making it 1.27 English miles or 2.05 km. There are some surviving milestones throughout the country showing distances in Irish miles, and despite being officially outlawed in 1824, there was still some residual use in the first few decades of the 20th century.


The Irish acre was 10 square Irish chains of 28 yards, making it 7,840 sq yds or 1.62 English acres. Again, despite it being outlawed from 1824, newspapers still advertised farmland and other properties using Irish acres up until the mid-20th century.


So, there you have it, a little look into some of the units St. Patrick would have used while going around converting everyone to Christianity. Maybe he had the odd méisrin to celebrate a successful conversion…

Sean-tomhas na hÉireann — Old Irish measures