"What do you have to say for yourselves?"
Translation:Céard atá le rá agaibh dóibh féin?
The FGB may not have cad atá but it does have several examples of cad tá which actually accords better with the most usual Munster pronunciation. When following a question word the relative particle is typically elided before tá and deir. There are additional examples of cad tá in the EID.
Céard also appears as its own headword, and is used in numerous examples in both the FGB and EID.
Le is a very flexible preposition. The 24th definition in the FGB entry for le says:
. 24. (With vn.)(a) (To express purpose) Tá scéal le hinsint agam duit, I have a story to tell you. An rud atá le rá agam, what I have to say. Le bia a cheannach, in order to buy food. Suim bheag le do chostas a íoc, a little sum to pay your expenses.
Two things, first as has been noted already dóibh is the 3rd person plural form of do. This sentence should read ...daoibh féin. Secondly, "to have to say for oneself" is an idiomatic expression and thus probably doesn't directly translate into Irish. I can find no attestation of any Irish expression ...le rá agat duit féin. I don't have an equivalent Irish idiom on hand but something like ...le rá agaibh ar bhur son féin captures the literal meaning well enough.
If attestation is what matters, can you quote your source for le rá agaibh ar bhur sonsa. For example, the FGB suggests "I spoke on your behalf" for Labhair mé ar do shon, which to my mind does not capture the literal meaning particularly well.
Dineen has 3 different examples of the phrase duit féin, explicitly translating it as "for yourself" in the first example:
ith é sin duit féin - "eat that (for yourself)" (this example is used in the definition of do and in the definition of féin)
fan istigh duit féin - "remain within doors"
is tú thamhachadh an siubhal duit féin - "you yourself would be the cause of your trouble"
Dineen mentions do as being the preposition used to signal a "dativus commodi" i.e. who benefits or loses from an action. All three of the examples you cite clearly fit this pattern. Had the sentence in question here been something like Abraidh paidir daoibh féin I also wouldn't have objected, because there it's clear daoibh féin is a "dativus commodi". I regard the English "to have to say for oneself" as idiomatic, and I don't see it as constituting a clear dativus commodi.
As for my own suggestion I'll admit it was done mostly by gut. The EID has ar a shon is féidir a rá go... for "in his defence, it may be said that..." which I think captures the meaning pretty well. I also think "What do you have to say on your behalf" has essentially the same meaning as the sentence in the exercise, the only difference is the former might be asked by a judge in court while the latter is probably said by a mother scolding her children. I said before and I'll say again that I don't guarantee my suggestion has the same pragmatics as the English, in fact I don't think it does. What I do think is that my suggestion is more likely than the direct calque from English featured here.
I don't think it's purely idiomatic, in the sense of having a meaning that's unclear from the sum of its parts (at least, no more so than any other prepositional verb in English). "To say something for someone" has a few meanings, including to speak in defence or support of them. "To say something for yourself" can thus be interpreted as speaking in your own defence, and "What do you have to say for yourself?" certainly has a connotation of demanding an explanation or excuse.
From the FGB entry for le:
le, prep. (Pron. forms:liom, leat, leis m, léi f, linn, libh, leo)(Prefixes h to vowel, becomes leis before article. S.a. an1. Combines (i) with possessive adjectives a, ár to form lena, lenár, (ii) with relative particles a, ar to form lena, lenar) With; to, for; by, against. …
…. 24. (With vn.)(a) (To express purpose) Tá scéal le hinsint agam duit, I have a story to tell you. An rud atá le rá agam, what I have to say. Le bia a cheannach, in order to buy food. Suim bheag le do chostas a íoc, a little sum to pay your expenses.
The only exercise that I can find in a quick search that contains a rá is Ní raibh mé chun é sin a rá.
In that sentence, the verbal noun has an object é sin, and the structure for using the verbal noun with an object is (object) a (verbal-noun).
The "to say" in this exercise doesn't have an object. The "to" in that sentence serves a different purpose.
bhí air rud éigin a rá - "he had to say something"
bhí rud éigin le rá aige - "he had something to say"