Ponsonby, Brabazon (1679–1758), 2nd Viscount Duncannon , 1st earl of Bessborough , revenue commissioner and lord justice of Ireland, was eldest son of William Ponsonby (1659–1724), 1st Baron Bessborough and Viscount Duncannon, of Bessborough, Co. Kilkenny, and his wife Mary (d. 1713), daughter of Randal Moore of Ardee, Co. Louth. Brabazon's father was a colonel in the army and served on the Williamite side at Derry in 1688–9, and, as MP for Co. Kilkenny, was one of those who offered to prove the articles of impeachment brought into the Irish house of commons in 1695 against Lord Chancellor Sir Charles Porter (qv). Brabazon followed in his father's footsteps and entered the army, being appointed captain of grenadiers by 1707. However, it was in politics that he made his name. He was first returned to parliament as MP for the borough of Newtown, Co. Down, at a by-election in 1705. His election had been assured, because he had gained control of the borough by means of his first marriage (c.1703) to Sarah (d. 1733), daughter of John Margetson (son of James Margetson (qv), archbishop of Armagh) of Sysonby, Leicestershire, and Bishopscourt, Co. Kildare, and widow of Hugh Colvill, who had owned the Newtown estate and had controlled the electoral borough. By this lucrative marriage Brabazon also came to own the Margetson lands at Bishopscourt and Sysonby. Initially his control of the Newtown borough stemmed from his position as guardian to his wife's son by her first marriage, Robert Colvill (c.1697–1749). Supposedly of unbalanced mind, Colvill at first agreed to leave the Newtown estate in his will to Brabazon's own second son, and Colvill's half-brother, John Ponsonby (qv) (1713–87), but in 1744 sold it at the urging of his mistress to Alexander Stewart. However, by that time Brabazon had long exercised control of the electoral interest in Newtown, and thereafter continued to so, despite the endeavours of Stewart to the contrary.
In his early years in parliament Brabazon was considered, along with his father, to be a tory. In 1706 he attended the birthday celebrations of the tory duke of Ormond (qv); in 1709 he voted against the money bill as part of the tory opposition to the whig court party; and in 1713, having been reelected for Newtown, he voted for the tory Sir Richard Levinge (qv), the unsuccessful court candidate for the speakership. In the same year Brabazon was appointed sheriff and governor of Co. Kilkenny, and in 1714 sheriff and governor of Co. Kildare. Returned to parliament for Co. Kildare in 1715, he shifted allegiance and attached himself to the interest of Speaker William Conolly (qv) in the commons. In 1720 Brabazon was one of those named in the second petition, in competition with that of Lord Abercorn (qv) for a charter to erect a bank in Ireland. With his father, he was listed as a potential subscriber for that bank, both men offering £5,000 subscriptions, for which they would have been entitled to three votes, the highest number possible, as shareholders. When the king's letter for a charter was issued in July 1721, Brabazon was included among the twenty-one commissioners for the bank. However, by late 1721, in light of a serious economic downturn, the collapse of the South Sea Company, and mounting opposition to the project in Ireland, he voted against the bank in the commons.
In 1722 Brabazon was granted the patent office of searcher of Waterford, Passage and Ross. In 1724 he succeeded his father as 2nd Viscount Duncannon, and in 1727, having been recommended unsuccessfully on two previous occasions in 1724 and 1726, was appointed to the Irish privy council. During the 1730s he was appointed as a trustee of the linen board of Ulster (1734) and a commissioner for the tillage act (1735). However, it was with the appointment of the duke of Devonshire (qv) as lord lieutenant in 1737 that the Ponsonby family began to move centre-stage in Irish politics, through the development of a long-lasting connection to the Devonshire family. The first outward sign of Brabazon's high standing in government was seen with his appointment to the revenue commission in early 1739, a post he held until succeeded by his second son John (qv) in 1744. Later in 1739, his eldest son, William Ponsonby (qv) (1704–93), married Devonshire's eldest daughter, Lady Caroline Cavendish. And, before 1739 ended, Brabazon was created earl of Bessborough. In 1741 his son William was granted a mark of favour rarely bestowed on an Irishman in the eighteenth century when he was appointed as the lord lieutenant's chief secretary, and in 1743 the connection was strengthened further when Brabazon's son John married another Devonshire daughter, Lady Elizabeth Cavendish. By the time Devonshire departed Ireland for the last time in 1744, the Ponsonby family had become one of the major political forces in Ireland, in competition with, in particular, the party led by Speaker Henry Boyle (qv). Reappointed governor of Co. Kilkenny in 1746 (which post he held thereafter until his death), in 1749 Brabazon was appointed as a marshal of the admiralty and later that year was created Baron Ponsonby of Sysonby, Leicestershire, in the British peerage. The designation for his British title came from the lands he had acquired in Leicestershire by his first marriage.
Brabazon played a central role in the power struggles between the leading political factions in Ireland in the 1740s and 1750s. Throughout this time he demonstrated his clarity of purpose, political acumen, and lack of scruples, for a time allying himself with the politically ambitious archbishop of Armagh, George Stone (qv), in opposition to Boyle, and later dropping the connection when it no longer served his purposes. At all times, his main concern was the advancement of his family. In the money bill dispute in 1753 he stayed firmly on the side of the government, and was rewarded in 1754 when he was chosen to serve as one of the lords justices appointed to the Irish government in the absence of Lord Lieutenant Dorset (qv). As a lord justice he urged a purge of the treasury because of the stance taken in parliament during the money bill dispute by certain treasury officials, in particular the teller of the exchequer, Nathaniel Clements (qv), against the government. Brabazon's interest in such a purge lay in his desire to get his son-in-law, Benjamin Burton, appointed to Clements's post of teller. However, Clements was also a man of influence, and managed to retained his office. In 1755 Devonshire's son and heir, the marquis of Hartington (qv), was appointed lord lieutenant. Following the chief governor's return to England after the parliamentary session of 1755–6, Brabazon, who had also been appointed vice-admiral of Munster for 1755–6, was once again appointed as one of the lords justices in 1756–7.
Brabazon died 3 July 1758 from a ‘stoppage of the bowels’ (HIP, vi, 79), and was buried at Fiddown, near Bessborough. He built Bessborough house on the family estate in Co. Kilkenny, while he also held lands in Co. Leitrim, and in 1737–8 purchased part of the Burlington estate in Cork, valued at £2,300 a year, which he made over to his son John on his marriage in 1743. In 1751 Brabazon was granted the fairs of Fiddown. Following the death of his first wife (1733), he married later that same year Elizabeth (d. 1738), eldest daughter and co-heir of John Sankey of Tennalick, Co. Longford, and widow of both Sir John King and John, Baron Moore of Tullamore. As with his first marriage, Brabazon's second marriage proved a lucrative arrangement, as Elizabeth brought with her an estate of £2,000 a year and £10,000 in cash. Brabazon had eight children with his first wife, of whom seven survived: William, who succeeded as 2nd earl of Bessborough; John, who became speaker of the commons and a leading undertaker in the late 1750s and 1760s; Richard, who like his elder brothers sat as an MP in the Irish parliament; Sarah, who married Edward Moore, 5th earl of Drogheda; Anne, who married Benjamin Burton; Elizabeth, who married William Fownes; and Letitia, who married Harvey Morres (d. 1766), 1st Viscount Mountmorres. All four son-in-laws sat in the Irish commons. In his final will Brabazon left an additional £10,000 to his second son, John, and £400 a year to his third, Richard. The eldest, William, was heir and executor, and apparently believed that these bequests in the latter will were additional to an earlier will of 1753, and were not legally enforceable, the second will not having been executed. However, he appears to have honoured his father's wishes.