It took some time, but today I learned the true identity of the woman who claims to be "Lady Grania Mary Mountbatten."
Her real name is Grania Mary O'Malley, the third child and only daughter of the late Hamilton O'Malley and his second wife, Eleanor de Pentheny O'Kelly, who was known as Sadie. Eleanor is also deceased.
Grania has four brothers, Charles, Patrick, Dermot and Owen (listed as unnamed son in Burke's Irish Family Records.)
Grania was born July 31, 1950.
Her identity was confirmed to me by a family member.