Numbers in Irish from náid to milliún: mutations, people and ordinals

Counting: a haon, a dó, a trí

When Irish counts in the abstract, as in a list, a telephone number or a sum, each number takes the particle a: a haon, a dó, a trí. Before a vowel the particle adds h, so one and eight are a haon and a hocht. The same forms run on into the teens, a haon déag, a dó dhéag, and into the tens, fiche a haon for twenty-one.

The tens themselves are fiche, tríocha, daichead, caoga, seasca, seachtó, ochtó and nócha, then céad and míle. Wiktionary records the older system of counting in twenties beside them: dhá fhichead, two twenties, for forty; trí fichid for sixty; ceithre fichid for eighty. Leathchéad, half a hundred, is an alternative for fifty. The decimal words are the ones the dictionaries give first.

Zero is náid. A million is milliún and a billion billiún, both listed by Wiktionary with the ordinals milliúnú and billiúnú.

Numbers in Irish before a noun: lenition and eclipsis

In front of a noun the number loses its a and three of the forms change: two becomes dhá and four ceithre, and one is aon with amháin after the noun. The noun itself stays in the singular and changes its first letter according to the number.

The Wikipedia article on Irish initial mutations lists the pattern. One to six lenite the noun, adding an h after its first consonant: aon bhó amháin, one cow; dhá theach, two houses; trí bhád, three boats; ceithre bhó, four cows; cúig phunt, five pounds; sé mhí, six months. Seven to ten eclipse it, putting another consonant in front: seacht gcapall, seven horses; ocht n-asal, eight donkeys; naoi gcat, nine cats; deich bpeann, ten pens.

The noun after the number is singular in all of these examples. English puts five pounds in the plural; Irish says the equivalent of five pound, cúig phunt. This is one of the first habits a learner of the numbers in Irish has to unlearn.

Beirt, triúr, ceathrar: counting people

People have their own numbers in Irish. Two people is beirt, three triúr, four ceathrar, and so on to deichniúr, ten people; the full set stands in the last column of the table. They are nouns in their own right, so beirt alone means two people and triúr acu means three of them.

A noun that follows is lenited, as in beirt fhear, two men, the example given in the Wikipedia article on mutations. For one person Irish uses duine, a person, and Wiktionary also lists aonar, alone, as the personal form of one.

The personal numerals reach into the teens as well: Wiktionary lists triúr déag for thirteen people and naonúr déag for nineteen.

Céad, dara, tríú: the ordinals

First is céad, second dara, third tríú; from four on the ordinal adds -ú to the number: ceathrú, cúigiú, séú, seachtú, ochtú, naoú, deichiú. Céad as first lenites the noun after it, an chéad bhliain, the first year, and the article before dara puts an h on a following vowel, an dara háit, second place.

Céad therefore has two meanings, a hundred and first, and context separates them: céad bliain is a hundred years, an chéad bhliain the first year. In figures the ordinals are written with the ending, 1ú or 1d for first and 2ú or 2a for second, as Wiktionary's list shows.