death of his father in June 1831, less than a month after he had regained his seat at the general election, he not only succeeded to his ancient title and large estates, but, as heir to his father's patriotic reputation, was the obvious choice of the county's liberal Catholics for the representation. Denis was elected unopposed to the seat, which he held until his death in 1847. His father had been an emancipationist and O'Conor continued in this tradition, always voting with Daniel O'Connell. O'Connell wrote to Denis a month after his father's death: The death of my most respected and loved friend, your father, was to me a severe blow ... How little does the world know of the value of the public services of men who like him held themselves always in readiness without ostentation or parade but with firmness and sincerity to aid in the struggles which nations make for liberty ... I really know no one individual to whom the Catholics of Ireland are so powerfully indebted for the successful result of their contest for emancipation ... His was not holiday patriotism ... No, in the worst of times and when the storms of calumny and persecution from our enemies and apathy and treachery from our friends raged at their height he was always found at his post. He was neither a prominent nor a particularly effective parliamentarian. His rather infrequent interventions were long-winded and pedantic, though always temperate. He 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 and Thomas Wyse; 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 and Charles Wood, joint Parliamentary Secretary to the Treasury, termed him ‘a gentleman, not wise but worth attention and civility’. On 6 July 1846 O'Conor was appointed Junior Lord of the Treasury in Lord John Russell's government; he was one of five Irish MPs who obtained junior ministerial posts as a result