Yon of Lesser Ireland
Iom, Yons d'Ierlande la Menor, Yons d'Yrlande
Yon appears to have been a subking in Gaul, captured by Bagdemagus during Arthur's war against Claudas. Bagdemagus added Yon's contingent to his own, and Yon himself seems not only to have gone over to Arthur's side, but to have distinguished himself as one of the best leaders on that side.
King Yon later turns up in Arthur's court, among the barons. Arthur appointed him head of the tribunal which judged and condemned Guenevere after her affair with Lancelot was exposed. When Arthur commanded his barons so sentence the Queen without delay, Yon reminded him that it was not the custom to pass any judgments after the hour of Nonne (about mid-afternoon).
Might Yon conceivably be identified with Yonec, the title character in one of the works of Marie de France?