You are correct the photo I posted was not of Edith, Marchioness of Londonderry.
It is of her eldest daughter, Lady Maureen Stanley.
The photo was taken at a fancy dress ball at Mount Stewart (the Londonderrys' Irish estate) in 1931.
The dress (and I presume the train) had originally belonged to Queen Alexandra who wore it to a Court in 1904.
Below are two of Lady Maureen's sisters: the Ladies Margaret and Helen Stewart at the same ball, but being considerably younger were not wearing grand jewels.