O'Shaughnessy's poem was (first?) set to music for contralto, choir and strings, by Edward Elgar in 1912. This piece of Edwardian proto-post-modernism features numerous quotes from the composer's own earlier works (including the Enigma Variations and the Violin Concerto, as well as Rule Brittania and God Save the King.
I don't know whether Aphex Twin sampled the Elgar, but they clearly should have done.