There's several issues I have here. First off, the agam makes it passive, something more like "That is what is being said by me." Also, I feel a direct relative clause is needed, not an indirect. The "that" is the direct object of the second clause (Think of it as "I am saying that"). I was always taught that if it's the direct object or subject of the second clause, you use the direct relative clause, where as if it's the indirect object (basically, if there's a preposition), you use the indirect relative clause, which is being used here (the difference is on the dependent/independent form of the verb in this case).
Sin atá mé a rá (That is what I'm saying).
Note: It's a rá because the object of rá (sin), comes before the VN.
Yes, it's the dependent form and therefore an indirect relative clause (and I'm still iffy about the use of an indirect relative clause here, honestly). But that should have nothing to do VN part of the sentence. I'm basing it off here where it talks about the structure of the VN for passives, and Learning Irish page 68, where Ó Siadhail gives examples such as Cé atá Cáit a phósadh for "Who is Cáit marrying?"
As well as, in this very course, the sample Tá an teach á thógáil for "The house is being built" (and you can use ag to add an agent).
I don't see any way this sentence can be interpreted as an active sentence. And, sadly, that's true for a lot of these in the Verbal Noun section. Now, if it didn't have the agent structure at the end (agam) it could be interpreted as "That is what is saying it,".
That depends on how you parse "that which I have to say". Do you mean "I have to say" in the sense of "I must say" (caithfidh mé a rá or ní mór dom a rá), or in the sense of possession - "I have something to say" (tá rud éigin le rá agam).
I am interpreting "that which" as "the thing that", which would give an rud go gcaithfidh me a rá or an rud atá le rá agam, but maybe you meant "that which" in some other way?
(It's also worth noting that sin a bhfuil is a phrase that means "that's it!" or "that's all" so sin a bhfuil á rá agam could also be "that's all I'm saying" or "that's all that I have to say")
Not really. The á rá in the exercise doesn't really match "to say", and with agam, is closest in meaning to "is being said by me" (a passive construction with an agent).
The NEID actually suggests sin a bhfuil le rá agam as a translation for "I rest my case" (equivalent to "that is what I have to say", or perhaps "that is all (that) I have to say"), so you're in the right ballpark, and when you get into the area of translating colloquial expressions, there's probably not much difference between "that's all I'm saying" or "that's all I have to say", but it is worth recognizing that the á rá ag.. construction has a slightly different meaning from the le rá ag... construction.
What does the "what" in the English sentence mean?
You're really not doing yourself any favours by looking for "literal" translations of phrases like this. You won't always use "what" to translate any of the elements of this sentence. sin a bhfuil on it's own can be translated as "that's it", "that's all there is" or "that's what there is" and probably others as well, with the choice of translation being left to whatever makes the most sense in English.
sin a bhfuil i gceist leis - "that's all there is to it"
sin a bhfuil fágtha den airgead - "that's all that's left of the money", "that's what is left of the money"
sin a bhfuil de chosúlacht ann - "there the resemblance ends", "that's what resemblance there is"
This is a somewhat colloquial statement that makes sense from the point of view of an Irish speaker, but can't easily be parsed by trying to look things up in a formal grammar.
The first thing to recognize is that sin a bhfuil is a phrase or idiom in it's own right, meaning "that's all" or "that's it".
The second idiom is that, while cad atá á rá agat? is, technically, a passive construction ("what is being said by you?"), it is used where English speakers say "What are you saying?", a demand for clarification. The á rá construction is used for both "saying it" (in the active voice) and "being said" (in the passive voice).
Ní thuigim leath dá bhfuil á rá aige - "I don’t understand half of what he says" (á rá aige - "being said by him")
Tá siad á rá os íseal - "they are whispering it about" (á rá - "saying it")
Is fada mé á rá sin leat - "I have been saying that to you this long time" (á rá sin - "saying that").
Normally, you would expect to see atá á rá agam, but because the first idiom sin a bhfuil already contains the indirect form a bhfuil, having the direct form atá immediately after it just sounds overly wordy (even if it might be grammatically sensible), so you end up with sin a bhfuil á rá agam, "(that's all) (being said by me)", meaning "that's all I'm saying" or "that's what I'm saying".