Thomas O'Hagan, 1st Baron O'Hagan, KP, PC (Ire), QC, was an Irish lawyer and judge. He served as Lord Chancellor of Ireland from 1868 to 1874 and again from 1880 to 1881.

The Most Illustrious Order of Saint Patrick is a dormant British order of chivalry associated with Ireland. The Order was created in 1783 by King George III at the request of the then Lord Lieutenant of Ireland, The 3rd Earl Temple. The regular creation of knights of the Order lasted until 1922, when most of Ireland gained independence as the Irish Free State, a dominion within what was then known as the British Commonwealth of Nations. While the Order technically still exists, no knight of St Patrick has been created since 1936, and the last surviving knight, Prince Henry, Duke of Gloucester, died in 1974. Charles III, however, remains the Sovereign of the Order, and one officer, the Ulster King of Arms, also survives. St Patrick is patron of the order; its motto is Quis separabit?, Latin for "Who will separate [us]?": an allusion to the Vulgate translation of Romans 8:35, "Who shall separate us from the love of Christ?"