Born the youngest child of Lord and Lady Dunsany, Isobel was raised alongside her older brother, Gordon, and older sister, Geneva. Gordon was killed in the Battle of Prestonpans during the Jacobite Rising of 1745; the Dunsanys took the loss very hard, and perhaps as a consequence cherished and spoiled their daughters even more than before.
In 1768, Isobel died of a "bloody flux" aboard a ship to Jamaica, where she and William intended to join Lord John, who was governor of the island at the time.
Described as small, round, bubbly and blond, with pale bluish-green eyes like her mother.
- Lord John Grey – Isobel marries Lord John around 1764, and they effectively become parents to her sister's son, William.
- Isobel is the Scottish form of the name Isabel, the medieval Occitan form of Elizabeth. Ultimately derives from the Hebrew name אֱלִישֶׁבַע (Elisheva; English Elizabeth) and means "my God is an oath" or "my God is abundance".
- Dunsany is the name of a title in the Peerage of Ireland, as well as a castle in County Meath. It comes from the Irish Dún Samhnaí.