There are several problems with the title. A portrait of Anne Molesworth, née Smyth, was listed in this document - plymouth.gov.uk/northgallery_labels_largeprint.pdf. Presumably that is this portrait. The label lists "Moleswoth - St. Aubyn Family Trust." If this was on loan to the Plymouth Museum for a temporary exhibition, then it is permanently in a private collection. The abcgallery lists the portrait's location as Pencarrow, Cornwall, UK. The title in the catalog lists her maiden name as Smyth, but the article for Anne Molesworth in thepeerage gives her maiden name as Adair. The article also lists her sons by name, but only states she had six daughters. There is a portrait for Louisa Anna Molesworth, possibly one of those daughters.
From artcornwall.org/features/Anne%20Molesworth_unframed via pinterest.com/alandom467/british-painters/.jpg.