Her article in thepeerage is here. A genealogical article has this about the portrait, “...On 17 Nov 1800, Frances Moore married THE RT HON JOHN ORMSBY VANDELEUR (1765-1828), Commissioner of the Customs for Ireland, MP for Ennis. She brought a fortune of £6,000 to the Vandeleur estate in Counties Clare and Limerick.
During her honeymoon in Rome, the above portrait was painted (the article shows a black and white photo of the upper part of this portrait - gogm). (The painting is by Gaspari Landi who trained the famous French artist, David. Source: Clare Library Fotoweb: Vandeleur Collection)…”
According to her geni.com article, she married Reverend Charles Moore Stewart before marrying John Ormsby Vandeleur.
From jeannepompadour.tumblr.com/post/147303784476/lady-frances-moore-1771-1833-by-gaspare-landi.
This is Directoire dress - a column with top and bottom separated by a belt or waist band.
Keywords: 1800, Landi, Frances Moore, Moore family, Vandeleur family, curly coiffure, modesty piece, U-necklne, elbow length tight upper puffed sleeves, Empire low waistline, close skirt, wrap