"They have big mouths."
Translation:Tá béil móra acu.
Béil finishes on a slender consonant so the adjective should be lenited. béala is an alternative plural too. Béil mhóra or less typically béala móra.
Gramadach na Gaeilge says "Attributively used adjectives agree with the noun in case, number and gender " and doesn't mention anything about lenition after slender consonant.
On the other hand, teanglann.ie provides masculine and feminine version for the nominative singular, but doesn't specify gender for the nominative plural just two versions, one for use with slender consonants. (I checked 4 or 5 adjectives, and they all seem to be this way.
So what is the rule? Adjectives of singular nouns follow the gender of the noun, but adjectives of plural nouns follow the caol/leathan?
An Caighdeán Oifigiúil says:
San uimhir iolra, séimhítear túschonsan aidiachta má chríochnaíonn an t-ainmfhocal iolra (firinscneach agus baininscneach) ar chonsan caol.
I'm going to attempt a bit if Duolingo Immersion here, and translate this, because I'm not sure that I could have handled this when I asked the question.
In the plural, the initial consonant of an adjective is lenited if the plural noun (masculine or feminine) ends in a slender consonant.
macliam2 is right, Duolingo is wrong.
It turns out that Gramadach na Gaeilge does in fact describe this rule. The line after "Attributively used adjectives agree with the noun in case, number and gender" says "Depending on the gender, case, number, they are declined and, if necessary, lenited" and links to this description:
lenition of an adjective is used:
... 5.after femin. and masc. nouns in the nominative plural, that end in slender consonants (only possible in the weak plural) e.g.: fir mhóra = big men