Mathgamain mac Cennétig
Mathgamain mac Cennétig (contemporary name - Mahon, son of Kennedy) was King of Munster from around 970 to his death in 976. He is the elder brother of Brian Bóruma and the ancestor of the McMahon family of County Clare in Western Ireland (Thomond).
Mathgamain was the son of Cennétig mac Lorcáin of the Dál gCais. His father died in 951 and is called king of Tuadmumu in the report of his death, and was probably succeeded by Mathgamain's brother Lachtna, who died in 953 and is not given any distinguishing title at his death.
The most common account as to Mathgamain death was that he was captured in 976 by Donovan (Donnubán mac Cathail), Tiarna (sept chief) of Uí Fidgenti, with whom the Dál gCais were rivaling for territory, and who were then in alliance with Ivar of Limerick. Donnubán handed him over to his most powerful ally Máel Muad mac Brain (Malloy, son of Brain) previous king of Munster, who had Mathgamain killed.
"Dalcassian historians say that Donobhan invited Mahon to a banquet in his house at Bruree and there, in violation of the laws of hospitality, seized and delivered him to the agents of his rival, Mulloy, who took him into the country of Cork and put him to death. Others of them say, that the Eoghanacht and Daleassian armies, after several battles, were face to face near Bruree, when the Bishop of Cork and the clergy interfered and proposed a conference at the house of Donobhan, who there betrayed Mahon to his enemy, receiving lands in Corea Laidhe as a reward.
On the other hand, several authorities tell the story either differently, or in such a way as renders the Daleassian accounts impossible. For instance, the Annals of Ulster, at the year 975, without any mention of Donobhan’s betrayal, say, "Mathgamhian (Mahon) son of Cennetigh, King of Caisel, was killid by Maelmhuaidh (Mulloy) son of Bran." And again under the year 977, "Abattle between Brian (Boroimhe), son of Connetigh, and Maelmhuaidh, King of Des-Mumha (Desmond), in which Maelmhuaidh was slain." Not a word of the slaying of Donabhan and his Danish allies by Brian, as the latter’s admirers relate.
Keating, narrating events under the monarch Donough, A.D. 954, says (O’Connor’s translation): "Mahon the King Munster..., was betrayed and seized by his traitoroys subjects in his own palace, and conducted as a prisoner, under a strong guard to Mac Broin, where he was, barbarously put to death by the people of that place." Keating did not attribute the death to Donobhan’s trachery.
Comerford, who wrote in 1751, gives the same account in his History of Ireland, page 191. He says that "Mahon, was seized by some conspirators in his own palace, and conveyed away to Meills Mae Broin, King of Oneachach where he was barbarously put to death by the people of that country, altho’ St. Collum Mac Cairagain (Bishop of Cork) solicited for his life."
It is quite probable that Donobhan, being senior of all the Eoghanacht Milesians, took part with Mulloy in his contest with Mahon for the crown of Munster, and that some vague reports of this were construed into treason by the bards of the opposite faction. Anyhow even at the present day, more civilized powers are not very scrupulous in time of war about the means of entrapping an enemy; but surely Mahon was not such an idiot as to walk unarmed, as it were, into his rival’s camp." 
- "Annals of Ulster AD 431-1201". CELT: Corpus of Electronic Texts. Retrieved 2007-10-24.
- Byrne, Francis John, Irish Kings and High-Kings. Batsford, London, 1973. ISBN 0-7134-5882-8
- Duffy, Seán (2004). "Brian Bóruma [Brian Boru] (c.941–1014)". Oxford Dictionary of National Biography. Retrieved 2007-10-22.
- Thornton, David E., "The genealogy of Gruffud ap Cynan" in Maund, Kerry L. (ed.), Gruffudd Ap Cynan: A Collaborative Biography. Boydell & Brewer, Woodbridge, 1996. ISBN 0-85115-389-5
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- Genealogical Memoir of the O'Donovans, formerly Kings of Ui Fidghente