Ní causes lenition as a verbal particle. If you don’t use a verb, ní is most likely a form of the copula. So, there’s no lenition. E.g. Ní anois = Not now. It’s an ellipsis of a full sentence, e.g.: Ní anois (atá mé ar meisce ) = (It’s) not now (that I’m drunk.)
Or: Ní go fóill = Not yet. -> Ní go fóill atá mé tagtha. = It’s not yet that I have arrived.