O'Conor, Denis (1794–1847), ‘The O'Conor Don’, politician, was born in May 1794, eldest son of Owen O'Conor (qv) (1763–1831) of Belanagare, Co. Roscommon, landowner and MP for Roscommon 1830–31, and his wife Jane (d. 1804), daughter of James Moore of Mount Browne, Co. Dublin. The O'Conors were prominent catholic gentry who counted among their ancestors two twelfth-century high-kings of Ireland, Tairdelbach (qv) and Ruaidrí Ua Conchobair (qv). They settled in Clonalis and Belanagare in the seventeenth century. The suffixes Donn (‘Don’) and Ruadh (‘Roe’) had been used since 1384 to distinguish contesting branches of the O'Conors.
In June 1831 on the death of his father, the sitting member for Co. Roscommon, Denis O'Conor was elected unopposed to the seat, which he held until 1847. His father had been an emancipationist and O'Conor continued in this tradition, always voting with Daniel O'Connell (qv). He was neither a prominent nor a particularly effective parliamentarian. His rather infrequent interventions were long-winded and pedantic, though always temperate. He was a founder member of the Reform Club and a member of the Stephen's Green Club. Through his wife Mary (m. 27 August 1824), daughter of Maj. Maurice Blake of Tower Hill, Co. Mayo, he was connected to the whig lords Charlemont and Leitrim, and was by disposition a conventional whig rather than a repealer, but family loyalty kept him in O'Connell's camp. In 1843 he was much drawn to the liberal–unionist alliance of William Smith O'Brien (qv) and Thomas Wyse (qv); he joined with them in obstructing the arms bill at the end of May 1843 and supported O'Brien a few months later in drawing up his remonstrance against British policy in Ireland, which served as a manifesto for the liberal unionists. However, when O'Brien was looking for signatures O'Conor demurred because, in Wyse's opinion, he was afraid of what O'Connell would say. O'Connell spoke well but blandly of O'Conor; Charles Wood, joint secretary to the treasury, termed him ‘a gentleman, not wise but worth attention and civility’ (Macintyre, 308).
On 6 July 1846 O'Conor was appointed a lord of the treasury; he was one of five Irish MPs who obtained junior ministerial posts as a result of the Irish party's alliance with the whigs. He died 22 July 1847. He was predeceased by his wife, and survived by two sons, both of whom became MPs – Charles Owen O'Conor (qv), MP for Roscommon 1860–80, and Denis Maurice O'Conor (1840–83), MP for Sligo 1868–83 – and by five daughters, all of whom became nuns.