Does anyone know why the noun ionnas is spelled with a double n in the CO, apart from in the phrase "ionas go"? The traditional spelling was in double n in both cases. Is it somehow conected to a lax and tense n's in Connacht? What about Dinneen's claim that "ionas go" is actually pronounced "nas go"?

Is it?

An foclóir beag only lists

ionas [ainmfhocal ]
d'fhonn is, i gcaoi is (ionas go mairfeadh sé, ionas nach rachadh sé ar strae).

Can you tell me what "ionnas" without the go means; I don't recall ever seeing it?

Aonghus, it is ionnas (as a noun) in Ó Dónaill's dictionary, surprisingly. Ionnas, as I expect you know (although it is listed as only a literary word) means "manner, nature", eg "sin ionnas an fhir" - "that was the manner of man he was".

Also: ionnas a ghaile - the nature of his valour; in ionnas taibhse, in the manner of a ghost; and fán ionnas sin, in that manner. They are the only examples given.

I didn't. I don't recall ever coming across it.

I suspect the "lit" is your answer: Ó Dónaill lists many words, and I think (but am not sure) that stuff listed with Lit is historical and therefore the historical form is given - so that people looking it up can find it.

"ionnas" gets 44 hits in the Nua-chorpas: but all of them are "ionnas go/nach"

Note that Dinneen gives "ionas" as a variant spelling. Both spellings may have been in use in the manuscripts.

Might the 'nas go pronunciation reflect second syllable stress where the first syllable unstressed vowel has been dropped? Like "anois" which is often "nis" in speech.

It might also be the case that ionas was pronounced with an initial schwa /ənəs/ like the prepostional pronouns ionam /ənəm//ənum/, ionat /ənəm//ənum/, etc. and was simply dropped in speech.