"Two girls are running."
Translation:Tá beirt chailíní ag rith.
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)."
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."
Yes, it's DeNTaLS-DoTS.
From An Caighdeán Oifigiúil:
9.4.3 AINMFHOCAIL LEIS NA hUIMHREACHA PEARSANTA
(a) Foirm na huimhreach pearsanta agus foirm an ainmfhocail ar a lorg
. (i) Má tá an focal beirt, agus é san ainmneach*, á cháiliú le hainmfhocal a thagann ina dhiaidh, is sa ghinideach iolra a bhíonn an t-ainmfhocal agus é séimhithe (seachas ainmfhocal a thosaíonn ar d, t nó s), e.g., beirt chaptaen; beirt chumadóirí; beirt drumadóirí; ag beirt bhan.
* Nó sa ghairmeach, sa chuspóireach nó sa tabharthach.
9.4.3 NOUNS WITH THE PERSONAL NUMBERS
(a) Form of the personal numbers and the form of the noun following it
. (i) If the word beirt in the nominative*, is qualified by a noun that comes after it, the noun is in the genitive plural, and it is lenited (except nouns that start with d, t or s), e.g., beirt chaptaen; beirt chumadóirí; beirt drumadóirí; ag beirt bhan.
* Or in the vocative, the accusative or the dative.