Ser Jacelyn Bywater, called Ironhand, an officer of the City Watch of King’s Landing and captain of the Mud Gate, is of a lesser branch of the house and was indeed too poor to afford knighthood. He came by it only as a reward, from King Robert Baratheon, for his part in the fighting at Pyke, in which he lost his hand, at the end of Greyjoy’s Rebellion.
Information about House Bywater that reveals spoilers from the books.