After Oxford he met and went into partnership with John Kasmin, and opened the Kasmin Gallery on New Bond Street, London in 1963. The Kasmin was a radical gallery for the time and showed British and American abstract and pop art. The gallery was described as "a beautiful space in New Bond Street designed for them by Ahrends, Burton and Koralek, with a curiously shaped white ceiling, white walls and a green-khaki rubberised floor. It was a space described by Kasmin as 'a machine for looking at pictures in'; those pictures, moreover, were prototypes of the new art. They looked as if they had been painted to be seen in museums: the space was designed for canvasses six feet square and upwards that would readily carry across a large room. The gallery thereby affirmed that painting had changed fundamentally: it was no longer being made to fit into drawing-rooms." Among the artists the gallery showed were Frank Stella, Kenneth Noland, Jules Olitski, Anthony Caro and most famously of all David Hockney. The Kasmin Gallery closed in 1972, with Kasmin going on to work in partnership with other London dealers up to the 1990s. Lord Dufferin was appointed a trustee of the Wallace Collection in 1973, and was also a trustee of the National Gallery, London and continued to support up-and-coming contemporary British artists. He also helped in the making of films about the pianistLiberace and the Playboy entrepreneur Hugh Hefner, as well as backing the controversial 1976 film Sebastiane, directed by the British filmmaker Derek Jarman. He was also a sometime director of the Guinness company, being a great-grandson of Edward Guinness, 1st Earl of Iveagh.
Lord Dufferin died on 29 May 1988 from an AIDS-related illness, aged 49. As there were no other living descendants in the direct male line from the 1st Marquess, the marquessate and the other peerages created for the 1st Marquess in the Peerage of the United Kingdom became extinct. The Barony of Dufferin and Claneboye, the family's older title in the Peerage of Ireland, passed to a distant kinsman. His sister married Robert Lowell in 1972, and they named their son Sheridan after his uncle. In the years immediately before, and especially after, her husband's death, Lady Dufferin developed new initiatives at Clandeboye, and today the estate has associations with a number of environmental organisations and projects, being a home for Conservation Volunteers Northern Ireland's biodiversity projects, training centre and tree nursery. The Northern Ireland branch of the Woodland Trust was established in 1998 in partnership with the Dufferin Foundation, and Royal Botanic Gardens, Kew have developed a blossoming relationship with Clandeboye since 2003. Lady Dufferin also returned to the art world, and has exhibited in galleries in London and New York under the name Lindy Guinness. She was also the inspiration behind the opening of the Ava Gallery at Clandeboye in 2004, which exhibits works by leading contemporary Northern Irish artists and an annual exhibition of museum-standard work by a major artist or group of artists.
Titles and styles
9 July 1938 – 25 March 1945: Earl of Ava
25 March 1945 – 29 May 1988: The Most Honourable the Marquess of Dufferin and Ava