Sir Denis’s fascinating archive consists of a vast accumulation of correspondence and academic notes relating to collections and individual works of art, in particular Italian art and artists. A substantial fototeca of photographs, prints and glass plate negatives is of significant research value. In addition, there are papers relating to his work with various committees and boards and his lobbying activity on behalf of the arts. There are papers relating to his extensive family history including details of the Mahon, Browne and Vesey families of Castelgar Co. Galway, and Westport Co. Mayo. The gift also includes furniture, silver and works of art.
Above is a page from Sir Denis's air raid diary from 1917 called ‘Some history about air raids which Denis was in’; an early example of a propensity to keep detailed records of everything. Here a seven year old Denis Mahon records an air raid on London in September 1917. As a child Sir Denis spent long periods with his nanny away from London because of the war. From 1915-1917, German airships, or Zeppelins, rained tonnes of explosives and incendiaries on London.