What long-running international legal and cultural battle did Lady Gregory champion following the tragic death of her nephew, Sir Hugh Lane, in the sinking of the Lusitania in 1915?

Correct answer: The return of his French Impressionist painting collection from London to Dublin.

Sir Hugh Lane left a codicil in his will bequeathing thirty-nine masterpiece paintings to Dublin, but because the document was unwitnessed, the National Gallery in London kept the collection. Lady Gregory made it her personal mission to wage a tireless, decades-long campaign against British museum authorities to secure their return. Her efforts, documented in her book 'Hugh Lane's Life and Achievement,' eventually led to a shared loan agreement between the institutions.