This is from An Músgraigheach, issue 6, Fóghmhar 1944, 21:
Normally you find "ní duart ná go" = I didn't say but that=what I did say was that. (Dictionaries all list "but that" as the meaning of "ná go", but all uses of "but that" are confusing in modern English, and so these glosses are not very helpful.)

But here we find: go nduart ná go ndéarfadh Diarmaid X. I think this means the opposite of ní duart ná go, i.e. "I said that Diarmaid would not have said X". Can someone confirm the meaning of ná go after a positive verb. Dictionaries are of no help here.

I hardly understand this nested sentence, to be honest, so only a guess:

There is (nithe) ná háiteochadh ... orm, containing a negation (ná = nach = which not)

Could the following part be perhaps a kind of an enumeration, enumerated by "ná" ("nor", here maybe better "or" in English)? It would be something totally different from "ná go" (CO: "nach")
So I'd translate: ... that I said or that Diamaid would say ... or that it is ...

Ó Sé in GCDh:
"Is féidir ná go а úsáid in ionad ná i ndiaidh príomhchlásal dearfach …"

But is beag and is annamh aren’t really "príomhchlásail dhearfacha" but rather part of Irish "príomhchlásail dhiúltacha".

I see. The original text is convoluted here, and the contextual meaning seems to mean that Diarmuid Ó Laoghaire wouldn't have written those things.

I've got a good reply from an expert on this, and this is what happened in this sentence. Dubhart is a misprint for dubhairt and both dubhairt and déarfadh are governed by Diarmuid:

Translation: there are certain things I could never be convinced of, e.g. that Diarmuid said or would say "agus gurbh é thusa a bhí ag déanamh an chleamhnais) or that he was behind the decision to add lenition to placenames like Ar Chnoc an Iúbhair.

[This is because it is ar Cnoc an Iúbhair in Muskerry Irish, with no lenition after ar. The problem in the other sentence in the book An Bhruinneall Bhán is that it should have been gur tusa and not gurbh é tusa.]