pˈɑːnsɑːnbi wʌzðɪ ˈɛldɪst sˈʌn ʌv vˈaɪkaʊnt dˈʌŋkænən hˌuː səksˈiːdᵻd æz ðə sˈɛkənd ˈɜːl ʌv bˈɛsbəɹˌoʊ ɪn dʒuːlˈaɪ wˈʌn θˈaʊzənd sˈɛvənhˈʌndɹɪd fˈɪfti ˈeɪt ænd lˈeɪdi kˈæɹəlˌaɪn kˈævəndˌɪʃ, dˈɔːɾɚɹ ʌvðə θˈɜːd dˈuːk ʌv dˈɛvənʃɚ.
Ponsonby was the eldest son of Viscount Duncannon (who succeeded as the 2nd Earl of Bessborough in July 1758) and Lady Caroline Cavendish, daughter of The 3rd Duke of Devonshire.