Anguish

King of Ireland and father to Iseult, beloved of Tristan. Demanding tribute from Cornwall, Anguish sends his brother-in-law, Morholt, to enforce the tribute in single combat with the Cornish champion (Celtic tribes often settled disputes by a battle of champions rather than field combat. In fact, they extol it's virtues in works such as the 'Tain bo Culainge'). Accepting the challenge, Tristan slays Morhalt.
Anguish is Angus or Aengus in Irish. A King Angus ruled in Munster towards the end of the fifth century. Emigrants from this part of Eire had been settling and founding colonies in Wales and southwest England and this may be the root of the tribute story- an attempt by the Irish for regional dominance.