"A bheil dà bhràthair agad?"
Translation:Do you have two brothers?
This is an issue that generates a lot of heat, with several people adamant that it is correct and annoyed that it is not accepted, and just as many people adamant that it is not correct English.
In particular there has been a considerable discussion here. I think it is worth repeating what I posted there
In what dialect? I am not aware that is used in standard American or British English. If it were, I think that this article on Wikipedia would mention that do-support is optional in this situation.
I think the fact that the Wikipedia article does not even mention it is really quite telling, as there is always someone happy to add colloquial variations - it is not restricted to describing prescriptive grammar. No one has come back stating in what dialect it is acceptable, or citing any published examples. I am afraid I don't think Duolingo is going to accept it unless someone does identify the dialect and provide evidence. I am absolutely certain than none of the Duolingo courses that teaches English to non-English speakers would accept it. D