Taken from PUL’s Notes on Irish Words and Usages.
Pé duine ’ thabharfaidh easonóir don rí ná ná tabharfaidh, ní do cheamalach mar thusa is cóir easonóir a thabhairt do (L. Mac Con, 18).
Here the first ná is “nor” and the second ná is “not.”
One may ask “why not nú ná tabharfaidh?” —Because the negative sense is not disjunctive. The negation is total. The meaning is:— it is not permitted you to insult him in the one case nor in the other case. The English language does not make this distinction between a disjunctive negative and a total negative.
If I say “I will not open the door whether John comes or does not come,” there is a disjunctive negative, i.e., there are two distinct contingencies in which I will not open the door. John’s coming is one; his not coming is the other. The two are disjoined in the Irish … Ní osclód an doras peoca ’ thiocfaidh sé nú ná tiocfaidh sé.
If I say “I will not open the door no matter who comes or does not come,” there is a total negative. The contingencies are taken together. In this case the Irish is … ní osclód an doras pé duine a thiocfaidh ná ná tiocfaidh.