He died, "from an extensive disease of the lungs".
Richard Dadd was born in 1817.
Richard Coyne - artist - died in 1990.
Richard King - artist - died in 1974.
Richard Cook - artist - died in 1857.
Richard Short - artist - died in 1916.
Richard Allen - abstract artist - died in 1999.
Patricia Allderidge has written: 'I Gatti di Louis Wain' 'The Bethlem Royal Hospital' -- subject(s): Bethlem Royal Hospital and the Maudsley Hospital, History 'The late Richard Dadd, 1817-1886' 'Bethlem Hospital 1247-1997 a Pictorial Record' 'Richard Dadd (1817-1886)' -- subject(s): Exhibitions
The cast of First Bite - 2011 includes: Richard Dadd as Rich Jon Gavaghan as Jake
See the link for 10 Richard Longs, hopefully one of them is the one you are asking about. If it is the artist Richard Long, he is still very much alive.
Richard Grayson - artist - was born in 1958.
Richard Slee - artist - was born in 1946.
Richard Coyne - artist - was born in 1926.