Bandearga might be a demonstration of “DeNTaLS DoTS”, where a word that ends in D, N, T, L, or S (such as bán) affects the lenition of a word that begins with D, T, or S (such as dearg). Another example of such a compound word where the second component isn’t lenited is feoilséantóir (“vegetarian”).
Because attributive adjectives agree with the noun in case, number and gender.
You have to use the plural form of an adjective after a plural noun, but that plural form is then modified by the spelling of the noun that it applies to, not the gender. So if the plural noun ended in a slender consonant, the plural form would have been "bhándearga". But "Cailíní" doesn't end in a slender consonant, so it's "bándearga".
(I originally referred to "the masculine plural form" of the adjective - I've updated this post to correct that).
Cailín is a masculine noun, believe it or not!
But, even if it was feminine, lenition only occurs after the singular definite article, so, for example, you get an chluas but na cluasa.
To complicate things even further, the plural of adjectives doesn't follow the same rule as the singular adjectives - lenition occurs if the plural noun ends in a slender consonant - cailíní ends in a vowel, so it is na cailíní bándearga, but it would be na fir bhándearga or na cait bhándearga, even though fear and cat are masculine.
This is ridiculous. Really? I've been asked the same exact questions over and over again in this section, with no change as I move from one section to the next...one lesson to the next has the same sentences over and over again. What's so important about the pink girls? Is this discussed often in Ireland?