Guinness, Mariga (1932–89), architectural conservationist and socialite, was born Hermione Maria-Gabrielle von Urach on 21 September 1932 in London, the only daughter of Albrecht von Urach (d. 1969), a member of the royal house of Wurtemberg from Schloss-Liechtenstein in southern Germany, and Rosemary Blackadder (d. 1975) from Berwick-upon-Tweed. She was gravely ill for the first few months, but had recovered when in 1934 her parents moved to Venice to work as journalists and then to Japan, where Albrecht worked for the German government. Her mother disliked Japan and became depressed. In 1937 Gabrielle (as she was known until 1950) was with her mother when she attempted to gain uninvited access to Emperor Hirohito's palace. Her mother was arrested, sedated, and deported, leading her condition to spiral into insanity (in 1941 she had a lobotomy). She spent the rest of her life in various private mental institutions and was unable to care for her daughter. Gabrielle was returned to Europe by her father, and was raised there by her godmother, Hermione Ramsden. They lived in Surrey and Norway, where she was educated by as many as seventeen governesses and, briefly, at two boarding schools.
In 1951 she met Desmond Guinness, son of Bryan Guinness (qv) and Diana Mitford. They married (1954) in Oxford. In 1955 they moved to Ireland, renting Carton House, near Maynooth, Co. Kildare. They had two children, Patrick (b. 1956) and Marina (b. 1957). In 1958 a shared love of Georgian architecture prompted them to purchase Leixlip Castle and reestablish the Irish Georgian Society. They campaigned for the protection and restoration of many architectural treasures: the Conolly Folly, Dromana Gateway, Mountjoy Square, and the Tailor's Hall. Most ambitiously, they bought (1967) Castletown House, Co. Kildare, with the intention of protecting it and creating a centre for the Irish Georgian Society.
Throughout the 1960s Leixlip Castle was a mecca not only for those interested in architecture, but for European socialites. Mariga was an exotic and sometimes glamorous hostess. By 1969, however, their marriage was in decline. She left for London and then Glenarm, Co. Antrim, to be with Hugh O'Neill (qv), later Lord Rathcavan. Desmond began a relationship with Penny Cuthbertson, whom he later married. When Mariga's liaison with O'Neill ended she again lived in Leixlip Castle for a time, but her marriage was over. A divorce settlement was finally concluded in 1981. Following a period in London, she rented Tullynisk House, near Birr, Co. Offaly, in 1983. By then she was no longer at the centre of restoration activity, and her last years were increasingly characterised by loneliness and heavy drinking. She died 8 May 1989 while returning to Ireland from Wales on board a ferry. A massive heart attack was complicated by an injection of penicillin, to which she was allergic. She was buried at the site of her first restoration triumph, the Conolly Folly, near Castletown House, Co. Kildare.