In 1836 William Parsons married an heiress, Mary Field. The Countess of Rosse (as she became in 1841), was a remarkable woman who added greatly to the estate at Birr, Co. Offaly. Mary was very artistic, and took to photography soon after its invention. She set up an impressive dark room in the castle, fully equipped with chemicals and apparatus, where she developed and printed her own work. She took many fine photographs, including portraits of family members and of the large telescopes which were being built at Birr by her husband. These photographs played a key part in the restoration, in 1997, of William's great reflecting telescope, for many years the largest in the world.

Mary won the prestigious silver medal of the Photographic Society of Ireland in 1859, and designed furniture and gates for the estate. Sadly only four of the eleven children of William and Mary survived to adulthood, but one of them, Charles Parson, became one of the all time great engineers.