John Barnewall, 3rd Baron Trimleston, was an Irish nobleman, judge and politician. He was the eldest son of Christopher Barnewall, 2nd Baron Trimlestown and his wife Elizabeth Plunket, daughter of Sir Thomas Fitz-Christopher Plunket of Rathmore, Lord Chief Justice of the King's Bench in Ireland and his second wife Marian Cruise. He succeeded his father as 3rd Baron in about 1513. His father, like most of the Anglo-Irish aristocracy, had supported the claim of the pretender Lambert Simnel to the English throne in 1487. After the failure of Simnel's rebellion, he received a royal pardon.

Sir Thomas Fitz-Christopher Plunket (c.1407–1471) was a leading Irish lawyer and judge of the fifteenth century who held office as Lord Chief Justice of Ireland. He was an ancestor of the Duke of Wellington in the female line. His second marriage to the heiress Marian Cruise inspired the ballad The Song of Mary Cruys.