I think Séamus Ó Néill asked some questions about this. Unfortunately, dictionaries and grammar books don't properly describe the Irish language.
In my dictionary there is this:
Quote:
Often used where the sentence is not formally negative, but has an underlying negative sense: ansan do stadadh de bheith ag déanamh aon choimeád ar cad n-a thaobh gur tháinig sí uaidh ná ar cad ’n-a thaobh ná raibh sí ag dul thar n-ais, then they stopped holding back why she had come from him or why she was not going back [PUL1907].
The underlying negative sense here is that the person stopped doing something - they were not holding back some information, and so the next clause can be with ná and not nú.
But you will encounter quite regularly sentences that don't fit into this:
Quote:
Aon smuíneamh toilteamhail, briathar ná gníomh a deintear i n-aghaidh dlígh Dé
This is from Ua Laoghaire's catechism. The only thing I can surmise is that the "aon" adds a more indefinite sense that can then justify "ná" too. You wouldn't get the impression from reading Ó Dónaill's dictionary that Ó Dónaill had ever met a native speaker of Irish - that dictionary is lacking so much.
With no proper academic resources, it is difficult to say more. All Irish academics are busy with Middle Irish and insist the Caighdeán should answer all your questions on Irish. None of them are studying Gaeltacht dialects--and they are all contemptuous of anyone trying to do so...