"Who is your lawyer?"
Translation:Cé hé do dhlíodóir?
That’s how DL works. It by and by introduces new material mixed into stuff already learned so that you can draw your own conclusions about what’s going on. Not every single aspect of complex grammar can be dealt with in the sparse notes that DL offers. They give you the very basics; the sentences provide a variety of usages.
Apart from that, we have encountered the h affixed between to “colliding” vowels previously, e.g. Dé hAoine. So this is only a new variation of an already learned mechanism.
iad is plural, dlíodóir is singular.
Cé hé do dhlíodóir? - asking one person about one lawyer.
Cé hé bhur ndlíodóir? - asking more than one person about their shared lawyer (just one lawyer).
Cé hiad bhur ndlíodóirí? - asking more than one person about multiple lawyers. You might also write that as Cé hiad bhur gcuid dlíodóirí?
If you know for a fact that the lawyer is a woman, you could indeed say cé hí do dhlíodóir?, but you are asking for the name of a lawyer that you have already seen.
But where the gender of the person can't be assumed, cé hé is the default.
If you feel aggrieved by the gendered nature of such constructions, you can work around them by asking "who's is representing you?" - cé atá ag déanamh ionadaíochta duit? or maybe even cé atá i do dhlíodóir? (though that might imply that your lawyer is new or temporary).
Is í Sinéad mo dhlíodóir is the correct form for a response (at least if you allow for conventional gender naming conventions), even if the question asked is cé hé).