Ponsonby, William Brabazon (1744–1806), 1st Baron Ponsonby , politician, was born 15 September 1744 in Dublin, eldest son of John Ponsonby (qv), landowner, MP, and later speaker of the Irish house of commons, and his wife, Lady Elizabeth, second daughter of William Cavendish (qv), 3rd duke of Devonshire. On 14 November 1760 he was admitted as a fellow-commoner at Pembroke College, Cambridge, but there is no record of his graduation. The Ponsonbys were one of the major ‘undertakers’ who dominated Irish politics during the mid-eighteenth century, and William's parliamentary career was largely determined by the family's political calculations. He entered the Irish commons as MP for Cork city in April 1764, and in 1766 was appointed examiner of customs. He retained his seat in the general election of 1768 and followed his father's lead in opposing the administration of Lord Townshend (qv), who had attempted to curb the influence of the great Irish ‘undertakers’. For this, Ponsonby was dismissed as examiner of customs in 1769. He responded by opposing the vote of thanks to the king for continuing Townshend as lord lieutenant in 1771, voting against two motions by Sir Lucius O'Brien (qv) for retrenchment, and opposing the short revenue bill of 1772 and a bill on absentee taxation (1773). In 1774 he voted against the stamp act and a bill on catholic relief (1774), and was among the small group of MPs who voted in 1775 for a pro-American amendment in the response to the royal speech.
Returned as MP for Bandon Bridge, Co. Cork, in the general election of 1776, he remained in opposition until the appointment of his relation, the duke of Portland (qv), as lord lieutenant (1782). In the general election of 1783 he was elected MP for Co. Kilkenny, and retained this seat until raised to the peerage in 1806. In November 1783 he voted against the parliamentary reform bill of Henry Flood (qv). He was a staunch supporter of the whig Charles James Fox, but agreed to support the Rutland administration after the end of Fox's brief term as foreign secretary in 1783. In return for this he was sworn of the Irish privy council and appointed joint postmaster general (1784), the latter appointment being worth around £1,500 a year. He unsuccessfully sought to become speaker in 1785, and that year opposed Pitt's Irish commercial propositions. When his father died in 1787, William assumed a more prominent position in the Ponsonby interest, but his brother, George Ponsonby (qv), later lord chancellor of Ireland, was a more able politician and was regarded as the faction's natural leader. William could be a difficult man to deal with: Edmund Burke (qv) described him as ‘perhaps the most vehemently irritable and habitually irritated person I have ever heard of’ (HIP, vi, 104). During the regency crisis of 1789 the Ponsonbys supported Fox and his campaign to have the prince of Wales appointed as regent. As a result, William was dismissed as postmaster general by the lord lieutenant, the marquess of Buckingham (qv).
Determined to retrieve the speakership of the Irish commons for his family, after the election of 1790 William stood as a candidate for the election to the speaker's chair. John Foster (qv) won the election by 145 votes to Ponsonby's 105. He was a founding member of the Whig Club, established in Dublin in June 1789, and the Ponsonbys were key figures in establishing a whig opposition in the Irish parliament in the 1790s. By 1791 the Ponsonbys headed a faction of about twenty whig MPs, but the grouping lacked focus, and loyalties were constantly shifting in the light of differing political opinions and objectives. Ultimately, William could only rely on the loyalty of his brother; his own sons, John and George Ponsonby; his brother-in-law, Denis Bowes Daly (qv); and his nephew, Viscount Boyle. He remained an important player in Irish politics nonetheless, voting for the resolution of John Philpot Curran (qv) against the sale of peerages (1791) and the motions of Henry Grattan (qv) on free trade and the abolition of the Dublin police (1791). Calling for moderate reform to counter growing political disaffection, he introduced motions for parliamentary reform in 1793 and 1794 but these were defeated. By this time, he also accepted the need for catholic emancipation and in 1794 voted for George Knox's (qv) motion in favour. Pitt's coalition with the Portland whigs and the appointment of Earl Fitzwilliam (qv) as lord lieutenant opened the administration to the Ponsonbys: Fitzwilliam recommended William for the post of principal secretary of state for Ireland in 1795, but was recalled before the appointment was made, and Ponsonby soon found himself in opposition again. He introduced another parliamentary reform bill in 1797, but with the troubled state of the country it stood no chance of success and was roundly defeated. Ponsonby then retired to the country and, until plans for a legislative union were announced in 1798, took little interest in politics.
He was a leading opponent of the union and voted against it in 1799 and 1800. In January 1800 he joined with Lord Downshire (qv) and Lord Charlemont (qv) and produced a circular letter, calling on MPs to raise petitions in their constituencies opposing the union. Continuing as MP for Co. Kilkenny, after the union had passed he voted in the British house of commons against the extension of martial law bill in 1801. In March 1802 he opposed Manners Sutton's motion for an investigation into the prince of Wales's finances. He achieved some prominence in Westminster and often spoke on Irish issues. In June 1804 he spoke on the subject of the Irish budget, opposing the imposition of wine and tobacco duties, and in May 1805 he spoke movingly on the decay of Dublin since the union, voting again in favour of catholic emancipation in the same month. He delighted in opposing William Pitt, voting against his ministry in 1801 and protesting against his reappointment as prime minister in May 1804. The resumption of hostilities against France in 1803 was, he believed, a grave error, and for the rest of his career he consistently voted against Pitt's war policy.
During his career he was awarded several civic honours and was elected a freeman of Dublin (1768) and Fethard, Co. Tipperary (1771). He was a trustee of the linen board of Ulster (1783–1800) and a governor of the Erasmus Smith schools. In 1787 he was appointed deputy governor of Co. Kilkenny, and was given a captain's commission in the Iverk Volunteer Cavalry in November 1796. He was a keen horseman and foxhunter and kept what was perhaps the best hunting establishment in Ireland at his seat in Bishop's Court, Co. Kildare. During the last few years of his life he suffered from poor health and when the Foxites took office in 1806 his wife pressed for his services to the Foxite whigs to be recognised. He was raised to the peerage as Baron Ponsonby of Imokilly, Co. Cork, in March and took his seat in the lords on 25 April 1806. He died 5 November 1806 at his residence in Seymour St., London. His remains were returned to Ireland for burial.
He married (26 December 1769) Louisa Molesworth (d. 1824), fourth daughter of Richard, 3rd Viscount Molesworth (qv), by his second wife, Mary Usher. With his wife's inheritance they had an income of £20,000. They had five sons and one daughter. Their eldest son, John Ponsonby (1770?–1855), a diplomat, succeeded as 2nd Baron and became 1st Viscount Ponsonby, and their second son was Maj.-gen. Sir William Ponsonby (qv). Younger sons included Richard Ponsonby (1772–1853), bishop of Derry 1831–53, and George Ponsonby (1773–1863), MP for Co. Kilkenny (1806), Co. Cork (1806–12) and Youghal (1826–32), and a supporter of catholic emancipation. Their daughter Mary Elizabeth (1794–1861) later married Charles, 2nd Earl Grey (qv). In 1823 William Ponsonby's widow married William Fitzwilliam, later 4th Earl Fitzwilliam. A half-length portrait of William Brabazon Ponsonby by Thomas Lawrence is in Leicester Museum and Art Gallery; an oval portrait of Lady Ponsonby by Angelica Kauffmann was in the possession of the family.