O'Shaughnessy, Sir Thomas Lopdell (1850–1933), judge, was born 22 December 1850 in Dublin, eldest son of Thomas O'Shaughnessy, a solicitor (enrolled 1868) of farming stock, of Dublin and Mohill, Co. Leitrim, and his first wife, Mary Anne (sometimes shown as ‘Marion’ or ‘Maryannie’) Lopdell of Gort, Co. Galway. He was educated briefly at Belvedere College (March–July 1863), Sullivan's school in Dublin, and QCG. Called to the bar in 1874, and becoming a pupil of Francis McDonagh, QC, with whom he gained valuable experience, O'Shaughnessy quickly attracted the support of Lord Chief Justice James Whiteside (qv). Practising initially on the Connaught circuit, O'Shaughnessy subsequently transferred to the north-eastern circuit (1880). Taking silk in Ireland 1889, he was called to the English bar at the Middle Temple in 1894 and elected a bencher of the King's Inns, Dublin, the following year. Although a crown prosecutor for Belfast and Co. Antrim, he practised extensively at nisi prius and in defence during the land war. While not eloquent as a barrister, he had a direct manner and was a leading exponent of the art of aggressive cross-examination to bring down a lying witness, exceeding J. H. M. Campbell (qv) in invective, discovering villainies and horrors in his opponent's client with a freshness of horror and indignation. In 1897 he was among those closely involved in the proposal to establish the general council of the bar of Ireland, and a member of the committee elected to draw up its constitution. Financially, it was considered he had as large an income as any then earned at the Four Courts. Among his friends were both Edward Carson (qv) and Tim Healy (qv), both of whom were in positions of influence at times of O'Shaughnessy's preferment.
Appointed on the retirement of Sir Frederick Falkiner (qv) in June 1905, he became a popular recorder of Dublin, where he had jurisdiction over both the city and the county, and was elected an honorary bencher of the King's Inns the same year. O'Shaughnessy spoke of himself as ‘the citizens’ judge’. He earned a reputation for fairness and compassion and brought to the bench a wide knowledge of human nature and a knowledge, hardly ever at fault, of the law. Understanding of the first offender who was the victim of circumstances, O'Shaughnessy was intolerant of violent crime, and while he was firm in his suppression of the mala fide traveller, he was just and fair to the law-abiding publican, notwithstanding his committment to temperance. He was sworn of the Irish privy council in 1912, an event marked by a banquet at the Gresham Hotel at which an illuminated address was presented to him. In reply, O'Shaughnessy spoke passionately about housing standards and efficiency in the administration of justice in commercial affairs. He sat in 1916 on every day on which he should, and in the opinion of Sir John Ross (qv) came through the difficult period and stormy time of 1916–23 with great credit.
In the absence of the viceroy, O'Shaughnessy served as lord justice and general governor of Ireland with others during 1921. In 1922 he was presented by Tim Healy (newly governor general of the Irish Free State) with a full-length portrait of Isaac Butt (qv), attested to by a formal engraved plaque still on the picture. He was consulted by the attorney general, Hugh Boyle Kennedy (qv), for suggestions, prompted by his unique experience and understanding of legal affairs in Ireland, and the short note in reply was circulated to the judiciary committee (UCD Archives, Kennedy papers). With some initial concerns about his pension (NA, C2/104), he accepted appointment to the new high court of justice in June 1924, when the office of recorder lapsed. The appointment was widely welcomed, even by an anonymous republican in the court gallery at Green St. courthouse, Dublin; although George Gilmore (qv), another republican, regarded O'Shaughnessy as a vindictive hangover from the British regime (Uinseann Mac Eoin, Survivors (1980), 595).
O'Shaughnessy supervised the establishment of the central criminal court at Green St. and the transfer to it of the ongoing business of the assizes, recently abolished in the Free State. Verdicts given at three trials he conducted in July 1924 were appealed, resulting in a number being reversed due to the admission of hearsay evidence and inadequate instruction to the jury ( 2 IR 193;  2 IR 9, 81). Nevertheless O'Shaughnessy subsequently sat in the court of criminal appeal, delivering one of its reported judgements in July 1925 ( IR 269). Ever a self-effacing man and the oldest of those appointed in 1924, he quietly announced his retirement and sat for the last time in the supreme court on the eve of his 74th birthday in December 1925 ( IR 367). His resignation was noted by the cabinet in Dublin without comment (NA, C2/235; S4713). The knighthood for public services announced in the new year honours list of 1927 was conferred in London on 18 February 1927.
A lover of Airedales and collector of antiques, particularly snuff boxes, O'Shaughnessy married (1879) Catherine Jane, third daughter of William Trueman of Shrewsbury, England, and lived at 23 Mountjoy Square before moving to 64 Fitzwilliam Square, Dublin, in the late 1890s, and (briefly, following his retirement) at 12 the Boltons, London, a house which he had purchased for his daughter and in which he maintained rooms until his death. His wife (d. London, 1927) and two sons, Thomas (KC; d. 1928) and Ernest (d. 1915), predeceased him, both sons dying of heart disease. He died at his Dublin home on 7 March 1933, survived by his two daughters, and is buried in the family vault at Mount Jerome cemetery. An oil portrait of him as recorder of Dublin and a pastel, showing him in the robes of an Irish privy counsellor, are in family hands.
O'Shaughnessy's daughter Edith Marion was among the first women to enter TCD to read as a junior sophister, and graduated from Dublin University (BA 1906, MA 1920). Her daughter Victoire Evelyn Patricia (‘Paddy’) Bennett (DBE 1991) was educated at the Sorbonne; married Sir Julian Ridsdale (a nephew of Stanley Baldwin, and MP for Harwich 1954–92); and served as chairman of the Conservative MP Wives Association, 1978–91.