Mason, (William) St John (1773?–1857), barrister and United Irishman, was from Co. Kerry, the only son of John Mason of Kinsale, Co. Cork, and Elizabeth Mason (née Austin). He was a first cousin of Thomas Addis Emmet (qv) and Robert Emmet (qv), as his father was a brother of their mother, Elizabeth. Graduating from TCD (1794), he entered the Middle Temple (1800), and was called to the bar on 17 March 1803, aged 30. Despite the close family connection, he would recall in later years that he had only known Robert Emmet from their Trinity days, when they were in constant competition during examinations and College Historical Society debates. An anonymous informer claimed he was a man of fortune and a United Irishman, representing Co. Kerry. After the arrests of the Leinster United Irish directory (12 March 1798), he went to live in Wales. Early in 1799, unobserved and one of a group of Irish law students in London, he began to move in radical circles and liaised with the many United Irishmen there, including William Putnam McCabe (qv) and George Palmer. They sailed to Hamburg in August 1799, and with the help of Lady Pamela FitzGerald (qv) travelled on to Paris. Mason joined the United Irish committee there, and on 13 September was listed among the refugees seeking relief from the French government, as an active member of the new home organisation who had been forced to flee Ireland or meet with certain death.
However, he returned to pursue his legal career. His political agitation and connections to Emmet placed him on a list of thirty-nine persons to be detained under suspicion, compiled on 10 June 1803 by the under-secretary, Alexander Marsden (qv). Around this time he lived at a boarding house at Seapoint, south of Dublin. He was not arrested until August, while embarked on his first circuit in Munster, at an inn in Nenagh. On him were several intimate letters from a female correspondent in London, which graphically alluded to ‘a longing . . . to tear flesh and draw blood’ (Geoghegan, 188). The authorities interpreted the gothic eroticism as an encrypted message from Emmet, and Mason was committed to Kilmainham gaol on 9 August and placed in a cell next to his cousin. He offered George Dunn, their gaoler, £500 for a key to Emmet's cell, and the same amount once the prisoner had fled the country, but Dunn betrayed him. Dublin Castle became convinced that Mason was a conspirator when another plan to free Thomas Russell (qv) was discovered. His father was interviewed, and energetically condemned how his nephew had seduced many unfortunate men by his radical beliefs, leading them to their deaths. Mason, too, condemned the rebellion unequivocally, but expressed no remorse at the attempt to help his cousin, who had such ‘a great and noble heart’, escape (ibid., 216). He remained in captivity and had Dunn pass on to Emmet the black velvet stock and hessian boots that Emmet wore at his trial. With other lawyers in Kilmainham he formulated the petition that exposed the petty tyranny of Dunn and the prison superintendent, Dr Edward Trevor (qv), presented to the viceroy on 12 August 1804.
After his release (1806) he engaged in a lifelong campaign against the government that had detained him, and others, without trial. He was supported by the parliamentary opposition, and on 17 May 1811 Richard Brinsley Sheridan (qv), MP, presented his petition to the house of commons (reprinted in Madden, 3:3 (1846), 221–4). Bitter and persistent, Mason exposed the prison abuses and ‘oppressions and atrocities’ practised by Trevor (Elliott, 250). A lengthy letter written from Bath as ‘Verax’ was published in The Times on 25 February 1842, and decried the ‘foulest perfidy’ practised by Dunn under the control of Trevor, ‘the haunting spectre’ (Geoghegan, 215). Becuase of his longevity, Mason was able to pass on many personal insights to R. R. Madden (qv) and others. He published Olithona, a ‘poem humbly attempted from Ossian’ in London in 1857, the year of his death (O'Donoghue, 304).