The play Ibsen was contemplating was "The Wild Duck," but he wouldn't start writing it until Spring of 1884, blaming a preoccupation with the "political complications in Norway" (as he put it in a letter to his publisher*) for his lack of progress.
Ibsen said he wasn't going to write about the "pure" flag, but he did, sort of. Gina makes herring salad for her guests.
I would guess that most modern American readers would miss this interesting reference. I, for one, knew nothing of the history of Norway's flag. To be really honest, I didn't really know about the history of Norway's independence either. So it was a delightful discovery to find herring salad "sandwiched" in Act III of a play that, otherwise, has nothing to do with Norwegian nationalism.
"Herring salad" (sildesalaten) is the nickname for the Norwegian flag from 1844-1899. The former Norwegian flag was combined with the Swedish flag to reflect the Norway/Sweden union (1814-1899), making a mish-mash flag which resembled herring salad. Personally, I don't see it. I "get" it -- the salad is a mish-mash and so is the combined flag. But I don't quite see herring salad when I look at the combined flag.
I understand the red (the beets), but the red was on the previous Norwegian flag. And there was already blue and white, too. Maybe the blue is supposed to be the herring, and the white is the onion? But they didn't call the old Norwegian flag "herring salad."
So when you add the Swedish flag, all you add, color-wise, is yellow. I'll guess that the yellow is the egg in the salad. Or the apple?
If you're the kind of person who trusts wikipedia, you can read the article on the Flag of Norway here and the union between Norway and Sweden here.
* Quoted in the Introduction to Michael Meyer's translation (Norton, 1961).