Aided Cheit maic Mágach

Background information

The History of Ireland (Geoffrey Keating), Volume 2

pp. 207-209 This Ceat was a valiant man and during his life he was an enemy and constant plunderer of the Ultonians. On a certain day this Ceat proceeded to Ulster to wreak vengeance as was his wont; and there was heavy snow at that time; and as he was returning with the heads of three warriors whom he had slain on that expedition, Conall Cearnach pursued him and seized him at Ath Ceit. They fought; and Ceat fell in the conflict; and Conall was severely wounded, and lapsed into a trance on the spot after he had lost a large quantity of blood. Thereupon Bealchu of Breithfne, a Connaught champion, came to the place of conflict, where he found Ceat dead and Conall on the point of death, and said that it was well these two wolves who had caused the ruin of Ireland were in so sad a plight. ‘That is true,’ said Conall; ‘and in retribution for all the injury I have inflicted on Connaught do thou kill me.’ Now he said this because he would give the kingdom of Ireland that some other warrior should wound him so that a single Connaught warrior should not have the renown of slaying him. ‘I will not slay thee,’ said Bealchu, ‘since the plight thou art in is almost as bad as death. However, I will take thee with me and apply remedies to thee; and if thou recoverest from thy wounds, I will fight thee in single combat, so that I may avenge on thee all the injury and affliction thou hast brought on Connaught.’ Thereupon he placed him in a litter and took him to his own house, and there applied remedies to him, until is wounds were healed.
But when Bealchu saw that Conall was recovering and his natural strength growing in him once more, he became afraid of him, and arranged for three warriors, his own sons, to slay him treacherously in bed by night. But Conall got a hint of this treacherous conspiracy; and on the night for which it was arranged that the sons should come to commit the murder, Conall said to Bealchu that he must exchange beds with him, else he would kill him. And accordingly Bealchu lay against his will in Conall’s bed, and Conall lay in Bealchu’s bed. And those three warriors, the sons of Bealchu, came to the bed in which Conall used to be and slew their father in mistake for Conall. Now when Conall observed that they had slain their father in mistake for himself, he sprang upon them and killed all three, and beheaded them and their father; and on the following day he took their heads to Eamhain in triumph. So far the murder of Ceat son of Magha and of Bealchu of Breithfne and his three sons.

Related poem online: Fianna batar i n-Emain
Whitley Stokes (ed. & tr.), “On the deaths of some Irish heroes,” Revue Celtique, 23, 1902, pp. 303-348.
Digital Edition at Archive.org (pp. 303-348)
The death of Cet mac Mágach (See Section 5), q. 19, p. 309, p. 321, p. 326, p. 337

Aided Ailella ocus Conaill Chernaig (Meyer)
p. 106 “Conall the Victorious: he was a man who never from his childhood so long as there was a spear in his hand went without the head of a Connachtman with him.”
“Cet mac Magach: from his childhood he never went without the slaughter of an Ulsterman.” (See Section 1)

List of Historic Tales in the Book of Leinster includes:
Aided Conaill (The Tragical Death of Conall), p. 587 (623)
Aided Fergusa (The Tragical Death of Ferghus), p. 588 (624)
Argain Belcon Breifni (The Slaughter of Belchu of Breifne), p. 590 (626)

Airec Menman Uraird Maic Coise (Byrne), Anecdota from Irish Manuscripts, Volume 2
List of the gnathscela Herenn includes:
Togail Bruidne Belchon Breifne, p. 44 (140), §5, line 8
Orgain Belchon Brefne, p. 47 (143), §8, line 4

R = The Rennes Dindshenchas (Stokes), Revue Celtique, 15-16, 1894-1895
M = The Metrical Dindshenchas (Gwynn)
S = Silva Gadelica (O’Grady), Volume 2
R: Brefne §149 (See Section 2), Part 4, pp.162-164, (‘Brefne’)
M: Bréfne (See Section 2), Volume 4, pp. 253-357, p. 443
S: Connachta (See Section 2), p. 509 (544)

Cóir Anmann: Fitness of Names (Stokes), Irische Texte, Ser. III.2
Conall Cernach §§251-252 (See Section 2), pp. 393-395, p. 423
Connachta §76 (See Section 2), p. 325, p. 414
Ulaid §245 (See Section 1), pp. 387-389, p. 422

Wikipedia
Ulster Cycle
Cet mac Mágach
Conall Cernach