Pól, you're hopeless.
Why is it "An" and not "Ar"?
Some of the irregular verbs have "an" instead of "ar" (and they keep "ní" instead of "níor" as well).
Why not ‘did you tell her that’?
how would you say then "did you say that with her" as in two people reciting something together?
But "léi" DOES mean "with her"! And isn't it pronounced "lay-hee"?
Yes, it does. Except when used with abair. In which case it refers to who you're talking with.
Léi can also mean “to her”, e.g. Éist léi! (“Listen to her!”)
Why is this wrong?
"did you tell her that"
and if it is wrong - how would "did you tell her that" be translated?
The entry here lists a different verb for "tell ," in Irish.
The only suggestion was "with her."
Be sure to report incomplete dictionary hints when you have the opportunity to do so.
Tell or say... is there a difference here?
Is there a difference between say and tell? Isn't it better to use tell when there is a complement?