"Two girls are running."
Translation:Tá beirt chailíní ag rith.
Why does this use "chailini" instead of the singular? The sentence "Three men are at work" uses "fear" instead of "fir". I don't understand the rule.
It actually is using the genitive plural. For strong plurals, such as cailíní, it's the same as the nominative plurals. For weak ones, such as fir, it's the same as the nominative singular, thus you get fear.
In the web version, the explanations for the Numbers lesson has some discussion of lenition under "3. Human conjunctive numbers." It's not very explicit, but the explanation implies that "duine" would sometimes be lenited, when it says "Note that duine in 1 and 11 is not lenited. For all other numbers of people you use the general conjunctive numbers as before (for example, trí dhuine dhéag thirteen people)."
I just had some girls doing something elsewhere and it wasn't lenited. What's the rule for plural girls not being lenited?
Does my reply to jimd_92 help you at all? I'm no expert, but lenition (and eclipsis) are dependent on various factors, such as grammatical form of the word, preceding words (prepositions, e.g.), etc. In this case, Duolingo's explanation implies that the genitive plural for girls (cailíní) would be lenited following "beirt."