William Brabazon Ponsonby, 1st Baron Ponsonby , was a leading Irish Whig politician, being a member of the Irish House of Commons, and, after 1800, of the United Kingdom parliament. Ponsonby was the son of the Hon. John Ponsonby, the Speaker of the Irish House of Commons and Lady Elizabeth Cavendish, daughter of the 3rd Duke of Devonshire. He was invested as a Privy Counsellor of Ireland in 1784. He served as Joint Postmaster-General of Ireland (1784–1789).

John Ponsonby, PC (Ire) was an Anglo-Irish politician.