John William married on 3 April 1877, at St George's, Hanover Square, London, England, his second cousin, Ernle Elizabeth Louisa Maria Grosvenor Burton (later, in 1906, by Royal Licence, Plunkett-Ernle-Erle-Drax), the only daughter and heir to Colonel Francis Augustus Plunkett Burton, Coldstream Guards, by Sarah Frances Elizabeth, younger daughter and coheir of John Samuel Wanley Sawbridge-Erle-Drax, of Charborough Park, Dorset.
In addition, Ernle, Lady Dunsany, was descended via her maternal line from James Drax, a young Englishman who, with a few hundred pounds, had sailed from the Port of London for Barbados in the late 1620s.
John William and Ernle Elizabeth had two sons, the author Edward John Moreton Drax, 18th Baron of Dunsany and Admiral Sir Reginald Aylmer Ranfurly Plunkett-Ernle-Erle-Drax (Charborough Park was left to Reginald, who changed his name to include Drax, the family who had formerly owned it, as well as including references to the Ernle and Erle families from which various of his mother's legacies to him came).
Lady Dunsany was a cousin of Sir Richard Burton, the great traveller, scholar, writer, and translator of the Arabian Nights.
His widow, from whom he was separated in his last years, and who, by Royal Licence of 20 December 1905, assumed the names of Plunkett-Ernle-Erle-Drax, died at her residence, Dunstall Priory, Shoreham, near Sevenoaks, Kent, England, on 28 February 1916.