Ponsonby, William (1704–93), 2nd earl of Bessborough , chief secretary for Ireland, was second (but first surviving) son of Brabazon Ponsonby, 1st earl of Bessborough, and his first wife, Sarah, widow of Hugh Colvill and daughter of James Margetson. Educated at home, he embarked on the grand tour, before returning to Ireland to enter the house of commons. Styled Viscount Duncannon, and supporting the whig interest, he was MP for Newtownards, Co. Down (1725–7), and for Co. Kilkenny (1727–58); his younger brother, John Ponsonby (qv), was speaker in the house of commons. He married (5 July 1739) Lady Caroline Cavendish, daughter of the 3rd duke of Devonshire (qv), lord lieutenant of Ireland; they had five sons and four daughters.
His wife was a great gambler, but Duncannon profited more from her family connections. On 19 June 1741 he became a privy counsellor, and was appointed chief secretary for Ireland, serving under his father-in-law. He also entered the British house of commons, and was MP for Derby (1742–54); he later sat for Saltash (1754–6), and Harwich (1756–8). He lost office in January 1745 after Devonshire was replaced, but by this time had established himself as an old whig at Westminster. Appointed a lord of the admiralty in June 1746, he held this office for ten years. When his brother-in-law became first lord of the treasury in 1756, Duncannon was promoted further, and was appointed a lord of the treasury.
On 4 July 1758 he succeeded his father as 2nd earl of Bessborough and on 23 November he took his seat in the British house of lords as 2nd Baron Ponsonby of Sysonby; he resigned from the treasury the following year. He was joint postmaster general in two administrations (1759–62, 1765–6), but withdrew from government after a dispute over place. He remained in opposition for the next decade, and played a leading role against the proposed absentee tax for Irish landlords in 1773. When the whigs resumed office in 1782 he declined to become involved, having lost interest in politics because of ill-health and old age.
He died 11 March 1793 and was buried in the Devonshire family vault at All Saints' Church, Derby. One contemporary noted of him that he ‘can never grow better or worse, or other than he is; it is incredible what nonsense he talks’ (G.E.C., Peerage, ii, 171). This weakness was never apparent in parliament for the simple reason that he never spoke. Nevertheless his family connections established him as an influential, though never important, political figure in Britain and Ireland. He was succeeded by his son Frederick Ponsonby (1758–1844) as 3rd earl of Bessborough.