GnaG also describes both ways:

23.1 Cardinal numbers 1–20
We are going to start overviewing some issues related to Irish numerals with listing of the numerals from 1 to 11 with nouns beginning with a consonant (capall) and with a vowel (ubh):

1. (aon) chapall amháin; (aon) ubh amháin
2. dhá chapall; dhá ubh
3. trí capaill; trí huibhe
4. ceithre capaill; ceithre huibhe
5. cúig capaill; cúig huibhe
6. sé capaill; sé huibhe
7. seacht gcapaill; seacht n-uibhe
8. ocht gcapaill; ocht n-uibhe
9. naoi gcapaill; naoi n-uibhe
10. deich gcapaill; deich n-uibhe
11. aon chapall déag; aon ubh déag
20. fiche capall; fiche ubh


We make constructions for 12 to 19 by adding déag to appropriate 1–9 forms […]

Let us notice consonant mutations: aon and dhá cause the lenition of a noun following them, and numerals from 7 to 10 cause the eclipsis […] Déag undergoes lenition after a word ending with a vowel, eg. trí huibhe dhéag […]

In constructions including numerals a special role is played by the indefinite pronoun ceann, cinn [… here goes a paragraph describing its usage]

Another construction of a numeral with a noun exists, that is a connection of a numeral, a word cinn with a preposition deᴸ and a noun. This way, alongside the construction Tá trí ba dhéag agam – I have 13 cows, there exists also Tá trí cinn déag de bha agam [… more examples]

Numerals 1, 2 and 20 are accompanied by singular form of a noun, eg.: aon bhliain amháin, dhá seachtain, fiche bó. Several nouns have special forms of the plural when put together with numerals, when the plural is needed (3–10). Also initial mutations are a little irregular.

Singular — Plural with a numeral

bliain — 3–6 bliana, 7–10 mbliana
mí [m’i] a month — 3–6 mhí, 7–10 mí
míle [m’ili] a mile, a thousand — 3–6 mhíle, 7–10 míle
pingin — 3–6 pingine, 7–10 bpingine
seachtain — 3–10 seachtaine
scilling [ʃk’iliŋ’] a shilling — 3–10 scillinge
uair — 3–5 uaire, 6 huaire, 7–10 n-uaire
lá — 3–10 lá

Numerals described above can refer both to personal and impersonal nouns. Although, as we will see later personal nouns have also separate forms. It needs to be noted at this place that numerals, more so than other parts of speech, allow a large range of alternative forms. The view presented above is idealized and one has to expect big variations of the forms in texts and in living speech.

The traditional way of counting tens between 10 and 100 is vigesimal (eg. 80 is four twenties), but some numbers have their own additional names. Odd tens are 10 added to a multiplicity of 20 (eg. 70 is 10 plus three 20s). The noun appears after the first numeral:
20 horses – fiche capall
30 horses – deich gcapaill fhichead [ihəd]
40 horses – daichead [dahəd] capall
50 horses – deich gcapaill agus daichead or leathchéad [laχ’iad] capall (lit. half-hundred)
60 horses – trí fichid [f’ihid’] capall
70 horses – deich gcapaill agus trí fichid
80 horses – ceithre fichid capall
90 horses – deich gcapaill agus ceithre fichid
100 horses – céad [k’iad] capall
1000 horses – míle [m’i:li] capall

We must add two more notes regarding multiples of twenty. The noun bliain year has a form blian [b’liən] with such numerals, eg. daichead, trí fichid, céad blian. Furthermore multiples of twenty are never accompanied by ceann. Compare the answer to a question An mó leabhar atá uait? – How many books do you want?: Daichead (no ceann) and Deich gcinn is daichead (with compulsory cinn).

Numerals céad and míle appear in plural preceded by an article and [with] a noun in singular, eg.: na céadta capall, na mílte capall.