It's the middle 1880s judging by Lady Hermione's mammoth bustle and the vertical drapery of her skirt. Lady Helen's dress is simpler using lace under-sleeves and modesty piece for decoration.
Keywords: Lady Helen Vincent, Viscountess D'Abernon, Hermione Wilhelmina Fitzgerald, Duchess of Leinster, Duncombe family, Vincent family, FitzGerald family, Viscountess, Duchess, British, curly coiffure, chemise, modesty piece, vee neckline, lace, quarter length notched under-sleeves, bodice, sweetheart neckline, strap sleeves, full skirt, modesty piece, sweetheart décolletage, elbow length bare shoulder notched over-sleeves, over-skirt, drapery, middle skirt, bustle, train, under-skirt, drapery, bracelets, fan