Here is an appearance of the story from April 1949, that I found via the British Newspaper Archive. This predates the July 1949 New Zealand Law Journal article quoted in the question, and may perhaps be the original printed source for the story, if ‘Creevey’s use of “I recall” can be trusted.
The tall, gaunt, bearded octogenarian, Serjeant Sullivan, K.C., defender of Roger Casement, is another who does not accept Mr. Attlee’s glib assurance that the creation of an Irish Republic makes no difference to its citizens who are resident here. Counting himself now a foreigner, he has felt compelled to retire from practice at the English Bar. His striking figure will be much missed. So will his fearless humour. I recall in particular a flash of irony from him which delightfully exposed the pedantry of an over-meticulous Judge. The Serjeant’s client, a poor illiterate Irish peasant, was claiming damages for personal injury. The Judge took the view that there were legal objections. “Mr. Serjeant,” he protested, “has your client never heard of the maxim, volenti non fit injuria? (an injury is not done to a person who consents). The Serjeant fixed the Judge with a steely eye. “My Lord.” he retorted in his rich brogue, “in the village of Ballymena, from which my client comes, it is almost the sole topic of conversation.” He had no further trouble with that Judge.
‘Creevey’ (1949). ‘A Club Causerie’. Yorkshire Post and Leeds Intelligencer, 29 April 1949, p. 2.
Note that in this version of the story Sullivan’s client comes from Ballymena (a real town in County Antrim) and not from the fictional Ballygullion. We must therefore understand the occurrence of “Ballygullion” in the New Zealand Law Journal article as being (among other embellished details like the plaintiff’s occupation and demeanour in court) an indication of memorization and recall: somewhere in the chain of transmission between the Yorkshire Post and the New Zealand Law Journal, someone could not remember the name of the town and substituted (accidentally or deliberately) the fictional name as a good enough equivalent.
The story collections Ballygullion (1908), The Shake of the Bag (1939), and Back to Ballygullion (1953) by Lynn C. Doyle are available on the Internet Archive, and you can check for yourself that they contain no mentions of Serjeant Sullivan, “volenti non fit injuria”, or the other elements from the ancedote.
Here are a few later retellings; note the variation in the location of the plaintiff’s home.
I am reminded of the story of a case from a little country town in Ireland, which had finally reached the highest court of appeal. One of the law lords at one stage interrupted counsel to suggest: “But surely, Mr. Sullivan, your client must be familiar with the maxim, Volenti non fit injuria”. “My Lord”, counsel replied, “in Ballygullion they talk of little else”.
George A. Johnson (1955). Osgoode Hall Lore: being an address given to The Lawyers Club in the convocation hall, Osgoode Hall, Toronto on April 14th, 1955, p. 3. Toronto: Law Society of Upper Canada.
There was once an Irish judge trying an action in tort between two semi-literate peasants from a remote comer of County Kerry. Suddenly turning to the counsel for one of them he said: “Surely your client is aware of the principle volenti non fit injuria.” “My lord,” replied counsel with deep seriousness, “up in the mountains where my client lives they talk of little else.”
Francis Cowper (1961). New York Law Journal 8-28-61, p. 4. Quoted in Eugene C. Gerhart, ed. (1969). Quote it! Memorable legal quotations, p. 261. New York: Clark Boardman.
Perhaps you recall the story of Sergeant Sullivan (the last of the sergeants), to whom the trial judge said, “Surely your client is familiar with the maxim, ‘volenti non fit injuria’.” Sergeant Sullivan replied, “My Lord, in County Kerry where my client lives, they talk of little else.”
Sanford D. Levy (1965). Review of Legal Ethics by Jerome E. Carlin. Bar Bulletin of the New York County Lawyers’ Association 23:5, p. 229.