How do I select and modify an NPC I can't see, via the console?
Use the BaseID to alter the character even if she's nowhere in sight. Birna's BaseID is 0001C187 according to UESP, so you could open the console and type: setessential 0001C187 1.
You could also use the console to move Birna to you, using the RefID:
prid 0001C18C then moveto player.