4.35 Séimhítear déag ar lorg guta
*sa fhrása dó dhéag: a dó dhéag; leabhar a dó dhéag; aon déag nó dó dhéag de bheithígh
*más cuid de bhunuimhir atá ag cáiliú ainmfhocal uatha é: aon lá dhéag; seacht mí dhéag; sé theach tábhairne dhéag.
Ach dhá bhróig déag; trí bliana déag; trí oíche Shathairn déag; an t-aonú (dóú, ... naoú) cú
déag.

déag after words in the singular ending in a vowel e.g.: seacht hata dhéag = 17 hats
déag after weak plural ending in a slender consonant (except cinn): e.g.: trí fir dhéag
déag is lenited after dó (but not after trí, sé, naoi) a dó dhéag = twelve

With the “teen” suffix, déag:
• There is lenition after singular nouns where the meaning is “eleven” where the noun ends in a vowel or where a feminine noun ends in /rʹ/ (at least): aon chasadh dhéag, aon chloch déag mine, aon fhear déag ar fhichid, aon lá dhéag, aon phingin déag, aon troigh déag, aon uair dhéag. A full set of examples where the noun ends in other slender consonants is not available.
• The cardinal number dó dhéag sees lenition of the teen suffix (but the other cardinals do not), and déag is also lenited after dhá+nouns in the dual ending in a vowel or where feminine nouns in the dual end in /rʹ/: dhá ainm déag (Ua Laoghaire’s translation of Exodus 28:21), dhá bhliain déag, dhá bhollóig dhéag, dhá chathaoir dhéag, dhá cheann déag, dhá chloich dhéag, dhá fhuinneóíg dhéag, dhá lá dhéag, dhá thobar déag (Ua Laoghaire’s translation of Numbers 33:9), dhá throigh déag, dhá uair dhéag. The available attested examples are a little confusing: why do we have dhá bhullóig dhéag but dhá throigh déag? These examples and the example of dhá chloich dhéag and dhá fhuinneóig dhéag suggest that morphologically apparent duals (i.e. declined as datives) ending in /hʹ/ and /gʹ/ also lenite déag in order that the slenderised ending be emphasised (while a feminine noun, troigh is not a morphologically apparent dative/dual). This would be an unwieldy rule, suggesting that there may simply be variation after slender consonants.
• The suffix is lenited after numbers over 3 used with a singular noun ending in a vowel: cheithre lá dhéag, sé dhuine dhéag.
• In theory, the suffix is lenited after plural nouns ending in a slender consonant, unless a confluence of dentals blocks lenition. Seacht n-uaire déag and naoi gcínn déag illustrate the non-leniting use. Real examples where the suffix is lenited after a plural ending in a slender consonant are hard to locate. It seems that few plural nouns intervene between the numeral and déag (Ua Laoghaire’s Bible has sé priúnsaí, but sé phriúnsa dhéag), but the lack of examples may also be partly because in Ua Laoghaire’s Irish fir could not be used before dhéag: trí feara déag, ocht feara déag (with devoicing of bhfeara), naoi bhfeara déag.
• There is no lenition with ordinals: an tríú lá déag.
•

With the “twenty suffix”, fichead (often erroneously spelt fichid in Ua Laoghaire’s works, which point is abstracted from in the examples below):

• Fichead is lenited after all singular nouns: aon mhíle fhichead, deich lá fhichead, dhá mhíle fhichead, gach rí de dhá rí fhichead. One might have expected nouns that end in a broad consonant not to show this pattern, but we find: ocht gcúbat fhichead, cheithre leathanach fhichead, deich ngrád fhichead. This may be because, unlike déag, fichead does not begin with a dental consonant, and so the pattern of lenition never became confused by considerations of dental cadad.
• Fichead is lenited after plural nouns ending in a slender consonant: cheithre bliana fichead, trí cathracha fichead, deich cínn fhichead, chúig cúbait fhichead, deich saoir fhichead de phríomh-shaoraibh Éireann, na seacht suíocháin fhichead, cheithre n-uaire fichead. Although cínn, cúbait, saoir and suíocháin are shown here in the plural ending in slender consonants, there seems to be a reluctant to use some plurals between the numeral and the “twenty” suffix. Hence we find deich maca fichead in Ua Laoghaire’s translation of Judges 10:4. Somewhat cryptically, Ua Laoghaire explained a similar usage with fear as follows:

Deich feara fichead (Sg. III. 299). Why not deich fir fichead? Because usage so ordains it. [Notes on Irish Words and Usages, 37]

Note this in the dative plural:

Iad ’na bpriúnsaíbh ar dheich cathrachaibh fichid. [Ua Laoghaire’s translation of Judges 10:4]

-Bh is a slender consonant, but does not lenite fichead. All identified relevant dative plurals in Ua Laoghaire’s manuscripts and printed works show the same pattern. It could be that fichead is in fact only lenited by a range of slender consonants and not all consonants, or that the dative plural is a general exception. A full range of the all the possible permutations is not available to determine this further.
• Fichead is not lenited after ordinals: an cúigiú lá fichead.