"Who is with her?"
Translation:Cé atá léi?
From Gramadach na Gaeilge:
The interrogatives cé = who, cad = what, céard = what, conas = how require a direct relative clause.
The interrogatives cá = where, cén fath = why, cén chaoi = how , as well as the combinations with prepositional pronouns cé/cad leis = with what, cé/cad air = on what , etc. require an indirect relative clause
So it looks like if the interrogative is made up of two parts (cá is from cén áit a), then you use the indirect relative clause, otherwise the direct.
It's the difference between "with him" and "with his" - in English, you use "her" in both positions.
I am walking with him - táim ag siúl leis
I am walking with his dog - táim ag siúl lena mhadra
I am walking with her - táim ag siúl léi
I am walking with her dog - táim ag siúl lena madra
léithe, (with a fada) is an old variant spelling of léi, but as it also means "greyness", you don't encounter it much nowadays - I wouldn't recommend you adding it to your writing vocabulary (though it's worth having in your reading vocabulary so that you'll recognize it in older texts or song lyrics).
The simplest way to understand this is to look at the answers.
"Who is the teacher?" - "Pól is the teacher" - Is é Pól an múinteoir
"Who is with her?" - "Pól is with her" - Tá Pól léi
If the answer uses the copula - Is é Pól an múinteoir - then the question uses the copula too. There is a "hidden copula" in cé, and the question Cé hé an múinteoir? is the copular question that matches the copular answer, where cé hé means "who is".
With the answer Tá Pól léi, you need to have tá in the question, and because there is already a copula in cé, you need relative clause, so you get Cé atá léi?, where cé atá is "who is".