Ponsonby, one of the major political connections of the 18th century. The Irish family was founded by Col. Sir John Ponsonby (1608–78), from Cumberland, who served under Cromwell and received land at Kildalton, renamed Bessborough, Co. Kilkenny. Its political eminence began with Sir John's grandson Brabazon Ponsonby (1679–1758), who attached himself to the duke of Devonshire, lord lieutenant 1737–45, under whom he became earl of Bessborough and 1st commissioner of the revenue. The alliance was sealed when two of Devonshire's daughters married Brabazon's sons William (1704–93), later 2nd earl of Bessborough and a reputed lover of George III's daughter Princess Amelia, and John Ponsonby (1713–87), who succeeded his father on the Revenue Board in 1744. In 1753 the Ponsonbys joined with Archbishop Stone to challenge the dominance of Henry Boyle. In the compromise which ended the resulting money bill dispute, John Ponsonby became speaker and a leading undertaker, until displaced by Townshend.

The family returned to political favour under the duke of Portland, lord lieutenant 1782, under whom John Ponsonby's sons William Brabazon Ponsonby (1744–1806) and George Ponsonby (1755–1817) became respectively postmaster‐general and 1st counsel to the revenue commissioners. Both were dismissed for their stand on the regency issue, and became prominent in the Irish Whig Party. As such they supported Catholic emancipation and parliamentary reform, although their commitment to both may well have been largely opportunistic. From 1808 to 1817 George led the Whig Party in the Commons of the United Kingdom parliament, and in 1806 was lord chancellor of Ireland during a brief period of Whig participation in government.

John William Ponsonby

(1781–1847), 4th earl of Bessborough, was a liberal and an improving landlord. Well regarded by O'Connell, he was a popular choice as lord lieutenant, but his administration (1846–7) was overshadowed by the Famine. His sister Caroline Ponsonby (1785–1828) married William Lamb, chief secretary 1827–8 and afterwards, as Lord Melbourne, a Whig prime minister; she achieved notoriety by her adulterous affair with Byron. Frederick Ponsonby (1816–95), the 5th earl, maintained his family's Whig/Liberal allegiance and gave his name to a commission of inquiry at the height of the Land War.