So I came her to see what this was, having never been introduced to it before. After reading all the comments it seems you would not say "She was not an actress" this way at all but I couldn't seem to understand from the comments how you would actually say "She was not an actress". BUT this is how you would say "She wouldn't be an actress" which is a sentences I don't think would ever need to be said anyway (I believe we would say "She would never be an actress"). Anyway, good to know.
Níorbh aisteoir í is the best way to say "She was not an actress".
While níor/níorbh can be either the past tense or conditional, context usually makes it clear which is most appropriate, and if you are talking about someone in the past tense, then the statement will be understood in the past tense.