I think that part was true, or at least one reads about it in the non-fiction side of the library.
The story of Grace is so interesting that I'm surprised Hollywood hasn't picked up on it. It's got many of the elements they look for: fighting against the social norms & superpowers of the day, action, women in battle (think Jennifer Garner), early historical evidences of feminist outlook, musical accompaniment (there are songs sung in Irish bars even today about Grace O'Malley) possibilities for sequels/spinoffs & merchandising, even precedence -- Mulan was quite successful.
I agree. Grace WAS Superwoman. Maybe someone could interest Mel Gibson in her story. He likes to do those historical costume epics.