Numbers in Irish

Overview

Numbers in Irish are more complex than in most European languages because they interact directly with the mutation system. At the A1 level, learning the cardinal numbers 1 through 20 and understanding their effects on following nouns is a practical priority — you need numbers for shopping, telling time, giving your phone number, and counting things.

The key rule to remember is that numbers 2 through 6 cause lenition on the following noun, while numbers 7 through 10 cause eclipsis. The number 1 (aon) also lenites, and 2 (dhá) is itself a lenited form that keeps the noun in the singular. Irish also has a separate set of counting numbers (a haon, a dó, a trí...) used when counting without a noun.

These mutation effects might seem demanding at first, but they follow a consistent pattern. Once you internalize the split between "2-6 = lenition" and "7-10 = eclipsis," the system becomes predictable.

How It Works

Cardinal numbers 1-10 with nouns

Number	Irish	Mutation	Example
1	aon... amháin	lenition	aon chat amháin (one cat)
2	dhá	lenition + singular noun	dhá chapall (two horses)
3	trí	lenition	trí chat (three cats)
4	ceithre	lenition	ceithre theach (four houses)
5	cúig	lenition	cúig theach (five houses)
6	sé	lenition	sé chat (six cats)
7	seacht	eclipsis	seacht gcapall (seven horses)
8	ocht	eclipsis	ocht gcat (eight cats)
9	naoi	eclipsis	naoi dteach (nine houses)
10	deich	eclipsis	deich gcapall (ten horses)

Counting numbers (without nouns)

Number	Irish
1	a haon
2	a dó
3	a trí
4	a ceathair
5	a cúig
6	a sé
7	a seacht
8	a hocht
9	a naoi
10	a deich

Numbers 11-20

Numbers 11-19 use the noun + déag pattern: aon chat déag (eleven cats), dhá chat déag (twelve cats). Twenty is fiche.

Examples in Context

Irish	English	Note
aon chat amháin	one cat	Lenition + amháin
dhá chapall	two horses	Lenition, singular noun
cúig theach	five houses	Lenition (2-6)
seacht gcapall	seven horses	Eclipsis (7-10)
a haon, a dó, a trí	one, two, three	Counting form
deich bpunt	ten pounds	Eclipsis
trí bliana	three years	Lenition + special plural
ceithre lá	four days	Lenition
ocht n-uaire	eight times	Eclipsis before vowel (n-)
fiche duine	twenty people	No mutation after fiche

Common Mistakes

Forgetting to lenite after 2-6

• Wrong: trí cat
• Right: trí chat
• Why: Numbers 2 through 6 trigger lenition. The c of cat becomes ch.

Using plural nouns after dhá

• Wrong: dhá chapaill (plural)
• Right: dhá chapall (singular form)
• Why: After dhá (two), the noun stays in the singular form, even though the meaning is plural.

Mixing up counting and noun-counting forms

• Wrong: a trí cat (mixing counting form with a noun)
• Right: trí chat (drop the "a" when a noun follows)
• Why: The "a" prefix is only used when counting abstractly (a haon, a dó, a trí), not when counting nouns.

Practice Tips

1. Practice counting objects around you: "trí chathaoir" (three chairs), "cúig fhuinneog" (five windows). This links numbers to mutations in a concrete way.
2. Learn the split rule as a rhyme: "Two to six, lenition sticks; seven to ten, eclipsis then." This covers the most important pattern.

Related Concepts

• Time and Dates — uses numbers for clock time, days, and months
• Ordinal Numbers and Quantities — first, second, third, and quantity words