She was neither born a Lady, nor did she marry a Knight or noble. But her husband, Thomas Charles Morgan, was knighted during their marriage. According to her Wikipedia article, she is a well-known Irish writer.
From nationalgallery.ie.
Lady Morgan wears an early post Napoléonic dress with a high waistline and short puffed sleeves in this Berthon portrait.
Keywords: 1818, Berthon, Sydney Owenson, Sydney Morgan, Lady Sydney Morgan, Irish, curly coiffure, ribbon headdress, bateau neckline, quarter length puffed sleeves, jeweled waist band, Empire waistline, close skirt, wrap, fur