Marhalt
Malory gives four names for Irish kings. Two, Agwisance and Anguish, probably variants of a single name and to refer to the same man. The third Irish king is Ryons (Rience), also king of Norgales, etc. The forth is Marhalt.
Marhalt was the father of Sir Marhaus (according to Malory) and thus the father-in-law of King Anguish, who married Marhaus' sister. The king is mentioned in Malory X, 66, fighting in the Lonazep tournament, but most of the time he remains in the background, perhaps busily governing his territory.
The chronology of Malory is a little odd: When Marhaus fought Tristan (Tristram) he was the brother-in-law of the Irish King Anguish, yet only later does his father Marhalt, ascend the vacant throne.
The family tree shows Marhalt's descendants drawn from a variety of sources. One wonders if names such as Marhalt and Marhaus might preserve some genuine memory of the fifth-century King of Tara, Muircheartach I.