Thought I should mention: Most plurals in Irish actually do sound like the singular even though 'bean - mná' doesn't.
For all (?) other languages, Duolingo uses a machine voice - those can read out any sentence at all.
For Irish, they don't have a suitable machine voice, so they got a real person to record sentences. But the speaker didn't record every single possible sentence that Duolingo can create, so many sentences are without pronunciation :(
Especially unfortunate given that Irish pronunciation is not exactly intuitive to beginning learners!
According to Wikipedia, bean evolved via a form ben from a Proto-Celtic form benā.
I don't know anything about Old Irish but I imagine that what happened is that final vowels got dropped in the singular form but kept in the plural form, but that the plural form dropped the interior vowel for some inflection-related reason and that then the consonant combination bn- got turned into mn- for easier pronunciation.
The shift from ben to bean, on the other hand, is due to newer orthography more clearly marking slender and broad consonants.
They're two different present tenses, though - one is present simple and one is present continuous.
Irish has a very similar distinction into a present simple and a present continuous tense. (In fact, I think I've seen the claim that the continuous aspect in English may be due to Celtic influence.)
This course starts by teaching the present simple tense in Irish, so it should be translated into present simple in English. Present continuous will come later.