Two things are apparent in this portrait: 1) she was beautiful and 2) he was an excellent artist. This article names her and provides a little genealogy. Some of his works can be found here.
Upgrade image posted 7 April 2015 also from Wikimedia.
The Viscountess Meneses wears a gorgeous white dress made all the more striking with red trimmings with extremely restrictive shoulder line.
Keywords: 1862, Viscount Meneses as artist, Viscountess Meneses, Mac Mahon Pereira Guimarães family, Miranda Pereira family, Viscountess, Portuguese, straight coiffure, floral headdress, off shoulder straight neckline, modesty piece, pleated bodice, quarter length puffed sleeves, engageantes, crinoline, over-skirt, floral bodice ornament, bows, necklace, bracelets, wrap