McKenna, Theobald (1765–1808), catholic activist and lawyer, was born 6 December 1765 in Dublin, the fourth son of Edmond McKenna, a catholic merchant originally from Carrick-on-Suir, Co. Tipperary. He trained as a physician and in the 1780s was active in the campaign for the repeal of the penal laws, criticising protestant intolerance in an anonymous pamphlet, A review of some interesting periods of Irish history (1786). He was elected a member of the RIA in 1790 and his name also appears in the membership register for Masonic Lodge 190 on 3 November 1791. McKenna played a prominent part in the politics of the 1790s, joining the Catholic Committee in 1790. Dissatisfied by its moderation, in October 1791 he and 40 other committee members founded the Catholic Society of Dublin, with McKenna becoming its secretary. On 21 October 1791 the Society published the Declaration of the Catholic Society of Dublin to promote unanimity among Irishmen and remove religious prejudices, written by McKenna, demanding total repeal of the penal laws as a matter of right. The declaration caused a split in the Catholic Committee: its more conservative members, led by Lord Kenmare (qv), and the catholic hierarchy seceded. McKenna was in touch with the Society of United Irishmen, founded in Belfast in October 1791, and joined its sister society in Dublin on 9 November 1791. For a short period his ideas closely resembled those of the United Irishmen and he was recognised as a leading catholic radical. In Some thoughts on the present politics of Ireland (1792), McKenna regretted the ill effects of laws that had purposely divided catholics and protestants. Taking the opportunity offered by the relief act of 1792, he abandoned medicine for law, and was admitted to King's Inns in 1792, entered Lincoln's Inn in 1794, and was called to the Irish bar in 1796.
McKenna helped organise the Catholic Committee elections of 1792, making his influence felt in Tipperary and in Waterford, where his brother, Richard, was a priest. He himself was returned for Armagh and sat at the Catholic Convention held in December 1792 in Tailors’ Hall, Dublin. During negotiations between catholic delegates and Dublin Castle a split developed in the committee between radicals who sought full political rights and moderates such as McKenna who were prepared to accept less. In the end the moderates prevailed and agreed to a relief act that gave catholics the vote but did not allow them sit in parliament. McKenna's growing moderation can be explained by the fact that since the end of 1792 he had become alarmed at events in France. Even though he continued to attend some meetings of the United Irishmen and was present at the Volunteer review in Belfast on 14 July 1792 to celebrate the third anniversary of the French Revolution, he found most of the United Irishmen's opinions dangerously radical, and resigned from the society on 26 April 1793. That year he published An essay on parliamentary reform in which he advocated moderate reform and catholic emancipation but distanced himself from United Irish radicalism and expressed himself in favour of constitutional monarchy and the link with England. Subsequent political turmoil in France and the bloody insurrection of 1798 confirmed him in these views.
Anxious to pass the act of union after the rebellion, the Irish administration offered McKenna £300 to write a pro-union pamphlet. Despite the scorn of erstwhile radical colleagues, he accepted, believing that the union would finally solve the catholic question. In A memoir on some questions respecting the projected union (1799), he argued that legislative union would be followed by a decline in the influence of the protestant ascendancy and Orangeism, and by immediate and complete emancipation of catholics. He was, however, to be disappointed. In Thoughts on the civil condition and relations of the Roman Catholic clergy, religion and people in Ireland (1805), he strongly criticised the government's failure to couple union with emancipation, and advocated that emancipation should be accompanied by state payment of the catholic clergy and a role for the crown in the nomination of bishops. McKenna died c.14 December 1808 in his house in Great Denmark Street, Dublin.
He married Frances Laffan of Kilkenny, widow of Francis McDermott, a wealthy catholic merchant. They had a daughter, Catharine and a son, Theobald (1797–1859), a barrister who became assistant under-secretary for Ireland in 1821 and who married Julia Taaffe (1806–81) of Smarmore Castle, Ardmulchan, Co. Meath; Catharine married Julia's brother, Robert Taaffe (1789–1854).