Like English, there are several forms of numbers (for counting — one, two, three; for ranking things — first, second, third; etc). In addition, Irish has some special forms for counting people.

The standard 'counting numbers' are:

zero	a náid
1	a haon	11	a haon déag
2	a dó	12	a dó dhéag	20	a fiche
3	a trí	13	a trí déag	30	a tríocha
4	a ceathair	14	c ceathair déag	40	a ceathracha
5	a cúig	15	a cúig déag	50	a caoga/leathchéad
6	a sé	16	a sé déag	60	a seasca
7	a seacht	17	a seacht déag	70	a seachtó
8	a hocht	18	a hocht déag	80	a hochó
9	a naoi	19	a naoi déag	90	a nócha
10	a deich	20	a fiche	100	céad
				1000	míle
21	fiche a haon	101	céad a haon
22	fiche a dó	102	céad a dó
				200	dhá chéad
				300	trí chéad

The 'a' in front of number is often omitted. It is required, however, when counting out loud, when saying an address, and when calling out numbers, like a phone number. Using 'a' adds an 'h-' (remember lenition?) to numbers that begin with a vowel — aon (one) and ocht (eight).

Counting things requires a few changes.

Rule 1: The singular form of the thing being counted is used.
teangacha — languages
trí teanga — three languages

Rule 2: The numbers one, two, and four change form for counting things.

one	a haon	one door	doras amháin
two	a dó	two doors	dhá dhoras
four	a ceathair	four doors	ceathre dhoras

As you can see, there are some changes to the words being counted.

	words starting with consonants	words starting with vowels
numbers 2 through 6	lenition	no change
numnbers 7 through 10	urú (not l, m, n, r, s)	and -n

When counting things greater than 10, the name of the thing being counted is put between the numbers:

cúig	five	cúig phunt	five pounds
cúig déag	fifteen	cúig phunt déag	five pounds and ten
seacht	seven	seacht bpunt	seven pounds
seacht déag	seventeen	seacht pbunt déag	seven pounds and ten

Counting greater than 20 things can be done in one of two ways:

seasca is a cúig punt — 75 pounds, lit. seventy and five pounds, or
cúig punt is seasca — 75 pounds, lit. five pounds and seventy

Counting people is different yet again. The standard 'counting numbers' are not used for people.

1 person	duine	6 people	seisear
2 people	beirt	7 people	seachtar
3 people	tríur	8 people	ochtar
4 people	ceathrar	9 people	naonúr
5 people	cúigear	10 people	deichniúr

Rule 1: beirt (two people) causes lenition.
Rule 2: the singular forms of nouns are used for counting
Rule 3: after a people-counting number, clann (children of the family) becomes clainne.

Finally, ordinal numbers (first, second, third) are slightly different as well.

first	an chéad	sixth	an seú
second	an dara	seventh	an seachtú
third	a tríu	eighth	an t-ochtú
fourth	an ceathrúr	ninth	an naoú
fifth	an cúigiú	tenth	an deichiú

These cause a few changes, too.

	words beginning with a consonant	words beginning with a vowel
after 'first'	lenition (not d, n, t, l, s)	no change
after 2nd-10th	no change	add h-