Beresford, Marcus (1764–97), politician and lawyer, was born 14 February 1764 in Dublin, eldest of nine children of John Beresford (qv), commissioner of revenue and son of Marcus Beresford (1694–1763), 1st earl of Tyrone, and Anne Constantia (d. 1770), daughter of the comte de Ligondes of Auvergne, France. He entered Westminster school (13 February 1778), graduated BA from TCD (1784), and was called to the Irish bar (1786). Like all the Beresfords he amassed his share of sinecures: aged seven he became joint-taster of wines at Dublin port (1773–97) and was later counsel at law for Dublin port (1789–94) and first counsel to the revenue commissioners (1795–6). He became a KC in 1791. Elected MP for the family borough of Dungarvan, Co. Waterford (1783–97), he formed part of Lord Tyrone's parliamentary connection, which strongly supported the government. In parliament he was a confident speaker who, despite his youth, was never overawed by the reputations of other members. Close to John Fitzgibbon (qv), the lord chancellor (his uncle the Rev. William Beresford was Fitzgibbon's brother-in-law), Marcus shared the Beresford family's political conservatism and opposition to catholic relief and parliamentary reform. Often employed by his father on political business, he appears to have been an amiable man, whose friends ranged across the political spectrum. At TCD he had formed a friendship with Theobald Wolfe Tone (qv), and he assisted him in the early stages of his legal career. Although he disapproved of Tone's radicalism, in 1794 he used his family ties to intercede on his behalf after Tone had become embroiled with the French agent William Jackson (qv). Largely because of Beresford's intervention the government allowed Tone to leave Ireland for America in June 1795 after he had given Beresford a written narrative of his involvement with Jackson. Struck down suddenly with an intestinal illness (possibly appendicitis), Beresford died 16 November 1797 at the family home, Abbeville, Co. Dublin. His large funeral cortege in Dublin was well attended by the city's dignitaries, particularly his colleagues from the bar and the lawyers' yeomanry cavalry corps; he was buried in the family vault at Curraghmore, Co. Waterford.
He married (25 February 1791) Frances Arabella, daughter of Joseph Leeson (qv), 1st earl of Milltown; they had a daughter Elizabeth (c.1794–1856), who married Felix Sadbrooke of Surrey, and two sons: John Theophilus (1792–1812), killed in Spain, and William (1797–1883), MP for North Essex (1847–65) and secretary at war (1852).