Her very brief Wikipedia article is here.
From the Prado web Site.
This is an extremely simple dress with just a frill at the hem and sleeves for additional parts. Her chemise barely shows in this portrait.
Keywords: 1798, Esteve y Marqués, Joaquina Téllez-Girón y Pimentel, marquesa de Santa Cruz, Osuna family, de Silva-Bazán y Waldstein family, Marquise, Spanish, long curly coiffure, chemise, bodice, scoop neckline, elbow length tight sleeves, waist band, Empire waistline, close skirt, shoes