She was Tsar Alexander II's lover from 1866 until they married in 1880 a month after Maria Alexandrovna had died. Her Wikipedia article is here.
Posted to the Alexander Palace Time Machine Discussion Forum by Carolath Habsburg on 15 June 2011; spots in background removed with Photoshop by gogm.
Ekaterina wears a full-skirted dress dating this to the first bustle period of the early 1870s.
Keywords: Ekaterina Dolgoroukaya, Princess, mistress, Alexander II, Russian, straight coiffure, high enclosing neckline, rebers, ruching, long close sleeves, full skirt, over-skirt, tassles