Elizabeth Hay , Countess of Erroll ( 17 January 1801 â `` 16 January 1856 ) , an illegitimate daughter of William IV and Dorothy Jordan , married William Hay on 4 December , 1820 , at the age of 19 . Elizabeth Hay , Countess of Erroll ( 17 January 1801 -- 16 January 1856 ; born Elizabeth FitzClarence ) was the illegitimate daughter of King William IV of the United Kingdom and Dorothea Jordan . She married William Hay , 18th Earl of Erroll , and became Countess of Erroll on 4 December 1820 at age 19 .