How many viewers, in millions, were for the episode directed by Tawnia McKiernan?
CREATE TABLE "table1_12159115_3" (
  "series_num" real,
  "season_num" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "production_code" text,
  "u_s_viewers_millions" text
);
SELECT "u_s_viewers_millions" FROM "table1_12159115_3" WHERE "directed_by"='Tawnia McKiernan';