O'Hagan, Thomas (1812–85), lord chancellor of Ireland and 1st Baron O'Hagan, was born 29 May 1812 at Belfast, the only son of Edward O'Hagan, a Roman Catholic trader, and Mary O'Hagan (née Bell). Educated at the Royal Belfast Academical Institution, he was a successful debater at school and decided on a career in the law. Entering the King's Inns in 1831, he moved to Gray's Inn, London, three years later. Called to the Irish bar (1836), he practised on the north-east circuit and was appointed editor of the Newry Examiner (1836–40). He regarded Daniel O'Connell (qv) as ‘my great political benefactor, my deliverer and my friend’ (O'Hagan, 129) and supported him consistently in his newspaper. In 1842 he successfully defended Charles Gavan Duffy (qv), then editor of the Belfast Vindicator, in a seditious libel case brought by the attorney general, and increased his prestige through skilful oratory. He was friendly with several leading Young Irelanders, but did not share their staunch nationalism.
In 1845 he joined the Repeal Association, but was never fully convinced of its merits and preferred to work within the union to secure greater rights for catholics. Appointed assistant barrister for Co. Longford in 1847, he became a QC two years later. Recognised as one of the finest lawyers of his day, he enhanced his reputation when he successfully defended Fr Vladimir Pecherin (qv), a Redemptorist priest, in 1855 in a controversial ‘Bible-burning case’. Promoted to third serjeant in 1859, he was also elected a bencher of King's Inns. He became solicitor general for Ireland in 1861, and attorney general and a PC the following year. Invited by the liberals to stand for parliament, he was returned as MP for Tralee, Co. Kerry (1863–5), in a by-election in 1863. In the house of commons he was highly critical of the Conservative government and emphasised the fact that, despite emancipation, catholics had not been appointed to the higher political and legal offices. Perhaps in recognition of this, O'Hagan was made a judge of the court of common pleas in January 1865, resigning his seat in parliament. With the formation of W. E. Gladstone's Liberal government in December 1868 he was appointed lord chancellor of Ireland, the first Roman catholic to hold that office since the reign of James II (qv). Undoubtedly part of the reasoning behind his advancement was political, and O'Hagan was ignored by the ministry in its early years.
In 1870 Gladstone's land bill passed through the house of commons, and O'Hagan was angry that he had not been consulted about it in advance. Furious with Chichester Fortescue (qv), the chief secretary, he was determined to establish himself as a key figure of influence, and complained strongly that he had also been ignored on the peace preservation bill and the processions bill. To placate his hurt feelings, and give him added authority in Ireland, on 14 June 1870 he was raised to the peerage as Baron O'Hagan of Tullahogue, Co. Tyrone. A founding member of the Dublin Statistical Society in October 1847, he was elected president of the Statistical and Social Inquiry Society of Ireland in 1869. His address to the society in 1870 was on the land act; his role in the society, where he argued for social and judicial reform, was a significant influence in shaping the legislation he later proposed.
In 1871 O'Hagan introduced bills to reform the jury system and also improve the Irish laws related to mental illness. In this work he drew heavily on utilitarian principles, particularly the thinking of Jeremy Bentham. O'Hagan was a proponent of penal reform, and believed that prisoners should be released once rehabilitated. But despite his obvious ability there were some weaknesses in his legislation; and his jury act, his most important reform, had to be modified several times. Crucially, his attempts to end jury-packing and increase catholic representation in juries were ultimately unsuccessful. O'Hagan resigned as lord chancellor in February 1874 with the collapse of Gladstone's ministry. In 1880 the Royal University of Ireland was incorporated and O'Hagan was named as vice-chairman and became a member of its senate. He was reappointed lord chancellor in May 1880 but his second term was not as productive as his first and he resigned in November 1881 owing to his failing health. He was made a knight of St Patrick on his retirement.
He died 1 February 1885 after suffering a paralytic stroke at his home at Hereford House, Park St., London, and was buried at Glasnevin cemetery in Dublin. He married first (5 February 1836) Mary Teeling (d. 1868); they had one son, who died in childhood, and five daughters. He married secondly (2 August 1871) Alice Mary Townley; they had two sons and three daughters. In 1865 his daughter Frances married John O'Hagan (qv), a judge and former Young Irelander. His eldest son from his second marriage, Thomas Townley O'Hagan (1878–1900), succeeded as 2nd Baron O'Hagan. A statue by Thomas Farrell (qv) was erected in 1887 at the Four Courts, Dublin, but it was destroyed during the fighting in 1922. For many years a bust of O'Hagan stood at the entrance to the bar library of the Royal Courts of Justice, Belfast, until it was moved upstairs after refurbishments. A similar bust by John Watkins is in the National Museum of Ireland. O'Hagan's papers are in PRONI (D2777).