"They are cleaning the cat."
Translation:Tá siad ag glanadh an chait.
@fr224, when you rephrase it to be so...
Tá an cat á ghlanadh acu. Tá + Object + (do+a=á) + verbal noun + ag + subject. The verb is lenited because á contains the possessive 'a' referring to masculine word 'cat'.
Tá an nathair á glanadh agat = you are cleaning the snake. (feminine).
You can see how other sentences are not using the passive in English to translate similar structures, but the more natural active English wording... In the same module.
They do not translate 'tá sé do bhur n-ionsaí' as 'you are being attacked by him', but instead use a more natural 'he is attacking you', though they might accept both.
Just like a literal 'at them' is not natural. The passive + by is not one you'd use in English in the same mindset as 'á ghlanadh acu'.
So they should accept both. Eventhough if asking for a translation from passive + by English to Irish, then the 'á ghlanadh acu' would be the go-to choice.
Translations don't always work strictly both ways...
tá sé = "He is". tá sé do bhur n-ionsaí is not a passive construction, and it naturally translates to "he is attacking you" because they both describe what he is doing, not what is being done to you. It's an entirely different construction from tá an cat á ghlanadh acu, and Duolingo is quite consistent in translating á + VN + acu as "by them" (at least it is now - it used to have the same passive/active confusion).