"Tell her the truth."
Translation:Insígí an fhírinne di.
Yes, it matters. Imperative statements (such as this exercise’s translation and your other example) should have the indirect object before the direct object, so Inis dom d’ainm is correct, but this translation should have been Insígí di an fhírinne. Please report it when opportunity allows for you.
As of July 2020, it's not accepting "Insígí di an fhírinne", so I've reported it.
Not commenting in the expectation of any action, as it's my understanding that different people handle the error resolution & alternative answer acceptance end of things. Just documenting here for learners who come to this discussion with the same question.
Whatever happens, go raibh míle maith agat as d'fhreagra anseo. <3
It's not as straightforward as scilling suggests.
These are examples from the FGB that support the construction that Duolingo teaches:
Inis scéal dúinn - "tell us a story"
Inis seo dom - "tell me this"
Abair an dán sin dúinn - "recite that poem for us"
Abair a fhíor liom - "tell me truly"
From *Gramadach na Gaeilge:
Indirect objects are ... placed between subject and direct object, particularly in: ...
Always in imperative sentences:
e.g. Scríobh dom litir! = Write me a letter!
That said, the FGB does not provide examples for inis and abair with the indirect object before the direct object, it only provides examples with the do after the direct object - Inis scéal dúinn, Inis seo dom, Abair an dán sin dúinn.
In English, there is no way to know if the command "tell" is addressed to one person or to more than one person — at least without some context of which there is none here. So, Duolingo should (and does) accept both "inis" and "insígí" here. The problem that others have identified with the Irish sentence is the placement of "di," and whether it should come right after the verb or at the end of the sentence.