It was created in 1739 for Brabazon Ponsonby, 2nd Viscount Duncannon,[1] who had previously represented Newtownards and County Kildare in the Irish House of Commons.

In 1749, he was given the additional title of Baron Ponsonby of Sysonby, in the County of Leicester, in the Peerage of Great Britain, which entitled him to a seat in the British House of Lords.

His son, the third Earl,[5][6] represented Knaresborough[7] in the House of Commons as a Whig and like his father served as a Lord of the Admiralty.

In 1834, ten years before he succeeded his father, he was raised to the Peerage of the United Kingdom in his own right as Baron Duncannon, of Bessborough in County Kilkenny.

William Ponsonby, third son of the third Earl, was created Baron de Mauley in 1838 while Lady Caroline Ponsonby, only daughter of the third Earl, was the wife of Prime Minister William Lamb, 2nd Viscount Melbourne, but is perhaps best remembered for her affair with Lord Byron.

Sir Spencer Ponsonby-Fane, sixth son of the fourth Earl, was for many years Comptroller of the Lord Chamberlain's Department and was admitted to the Privy Council in 1901.

The present family seat is Stansted Park,[12] near Stoughton, West Sussex, acquired by the 9th Earl in 1924.

Located near the village of Piltown in the south of County Kilkenny, the house was sold by the 9th Earl in the late 1930s.

They have three children:[13] The baronies Ponsonby of Shulbrede and de Mauley are in the remainder for the earldom of Bessborough.