Katherine inherited the entire Dromana estate on 1 March 1664, at the age of three, upon the death of her father, who was the son of Sir Gerald FitzGerald, Lord of the Decies (died 1643) and Mabel Digby.
Her ambitious uncle Baron Le Poer, wanting to unite the Curraghmore and Dromana estates, arranged a marriage between Katherine and his own son and heir, John.
Two years later, Katherine, who had not wanted to marry her eight-year-old cousin John Le Poer, appealed to the Archbishop to grant her an annulment on the grounds that she had not freely consented to the marriage.
Her marriage to Edward Villiers was thus declared legal and valid, while her uncle was forced to give back her lands and renounce the title of Viscount Decies.
Shortly after the death of her former father-in-law, Viscount Grandison, Katherine was granted the rank of Viscountess by Royal Warrant issued by King William III of England on 6 January 1700.