[2] His younger brother was Robert Henry Southwell.
[3] Southwell was educated at Trinity College Dublin.
[3] In 1767, Southwell entered the Irish House of Commons for County Limerick, the same constituency his father had represented before, and sat for it until the following year.
[5] On 7 November 1774, he married Sophia Maria Josepha Walsh, third daughter of Francis Joseph Walsh, Comte de Serrant, and had by her four sons and four daughters.
[2] Southwell died aged 53 and was succeeded in his titles by his oldest son Thomas.