The arms are those of Wingfield impaling Stratford, as borne by Richard, 3rd Viscount Powerscourt (1730-1788).
Richard, 3rd Viscount Powerscourt succeeded his brother, who died unmarried at age 34 in 1764. In 1760 he married Amelia, daughter of John, 1st Earl of Aldborough. His father, the 1st Viscount Powerscourt, was responsible for remodeling the family seat at Powerscourt, Enniskerry, yet the 3rd Viscount is chiefly remembered for erecting the stone-cut townhouse in Dublin, on South William Street, between 1771-74 (Complete Peerage).
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Portrait of Richard, 3rd Viscount Powerscourt (1730-1788), by Robert Hunter