Plunkett, John Hubert (1802–69), lawyer and politician, was born in June 1802 at Mountplunkett, Co. Roscommon, younger of twin sons of George Plunkett, gentleman farmer, and Eileen Plunkett (née O'Kelly). He was related through his mother to St Oliver Plunkett (qv). Plunkett entered Trinity College Dublin (TCD) in 1819 and graduated Bachelor of Arts (BA) in 1823. He was called to the Irish bar in 1826 and later to the English bar. While working as a barrister on the Connacht circuit, his success, popularity, and support helped the O'Connellite parliamentary candidate, O'Conor Don (qv), to win Roscommon in the crucial election of 1830. Daniel O'Connell (qv) showed his gratitude by adding his influence in London to that of Plunkett's kinsman, Lord Fingall (qv), to secure Plunkett’s appointment as solicitor general of New South Wales – the first catholic to achieve high civil office in the colony. Plunkett married (1832) Maria Charlotte (d. 1895), née McDonougha; they arrived at Sydney in June 1832. The marriage was less than harmonious and remained childless.
The arrival of increasing numbers of free immigrants in New South Wales in the 1830s presaged the transformation of the old penal colony into a free society. It fell principally to Plunkett (appointed attorney general in 1836) to be an architect of that transition. His first contribution was designed to extend the knowledge of the law among the local magistrates and guide them to exercise their ‘most serious trust’ with justice. In 1835 he published, at his own expense, the first Australian practice book, An Australian magistrate. Extended with each edition, it continued to appear until 1911, but by the fourth edition (1881) the original authorship was omitted. In the book, Plunkett instructed magistrates that the aborigines were within ‘the king's peace’ and that murdering them was as much a crime as killing any of the king's subjects. Together with Governor George Gipps, he insisted on the enforcement of the law in 1838 when seven white men were hanged in Sydney for the murder of about forty aborigines at Myall Creek in northern New South Wales. It is believed to be the only time white men were executed for killing blacks in colonial Australia, although massacres of blacks by whites were plentiful.
He regarded the church act of 1836 as his most decisive contribution to the development of a free society. The act finally made it clear that the Church of England was not established by law in Australia, but Plunkett failed in his attempt to include Jews in its provision for equality among religions. A member (1843–56) of the executive council, he resisted moves to revive transportation in the late 1840s, supported a national system of education as first chairman of its board, and helped draft the bill that resulted in partial representative government in 1842. In 1853 he inadvisedly agreed to Wentworth's contentious proposal for an hereditary upper house in New South Wales; Plunkett's main objective – in which he succeeded – was to avoid an elected upper house.
Plunkett helped to establish the University of Sydney in 1850, sat on its first senate, and served as vice-chancellor 1865–7. He had retired as attorney general in 1856, but won a seat in the legislative assembly on several occasions and served briefly as attorney general in 1865–7 before becoming president of the legislative council. He published On the evidence of accomplices in 1863.
Thoroughly English in his understanding and application of the law, Plunkett remained Irish in his passion for the traditional music of his native land. He gave numerous lectures in Sydney on Irish music and made the lectures live by playing the airs on his much-loved Cremona violin.
His last public role as a leading catholic layman was as secretary to the second provincial synod in Melbourne in 1869. He died in East Melbourne on 9 May 1869 and was buried in the Devonshire Street cemetery, Sydney. His remains were later reinterred with his wife's at Waverley cemetery, but his name does not appear on the headstone despite his unique contribution to the development of the British legal system in colonial Australia.