Sir Thomas Southwell, 1st Baronet, of Castle Mattress (died 7 December 1680) was a high sheriff of County Kerry under the Protectorate.
Thomas was probably born at Castle Matrix, near Rathkeale, the fifth but only surviving son of Edmund Southwell and his wife Catherine Herbert.
[2] His grandfather, John Southwell of Barham, had come to Ireland from Suffolk during the reign of James I.
On 4 August 1662 he was created 1st Baronet Southwell, of Castle Mattress in the Baronetage of Ireland, by Charles II.
[12] The castle mentioned in the territorial designation stands on the left bank of the River Deel, west of Rathkeale, and is also called Matrix or Matras.