Cumhail, father of Fion, King of Leinster, and head of the Clan Baoisne, ruled the Fianna in the reign of Con of the Hundred Battles. While in Alba (the Highlands), checking the attempts of the Romans and other unprincipled marauders, circa A,D. 154, the Ard-Righ Con transferred his honours and dignities to his own Daltha (foster-father), Crimthan. Hearing this, Cumhail sped back in the "high-cornered, big-bellied, broad-sailed barque," landed at Inver Colpa (Drogheda), dispossessed Crimthan, and made raids on Bregia (the level plain from the Boyne to the Dublin hills), and on to Meath, and took spoils of cattle and slaves, male and female,and cloaks and bucklers, and well tempered glaives (Cloidhimh, pron. Chloive) and coats of mail, and chessboards and chessmen. Loudly complained the graziers, and farmers, and small chiefs to the king, and, in consequence, he sent swift messengers to Naas of the Kings, then the capital of North Leinster, ordering Cumhail to attend a national meeting at Tara, and answer for his outrages. On his refusal, Con summoned to his aid Goll Mac Morna, the best warrior in Connaught, along with the Ulster chiefs, Achy of the Red Neck, Iomchy of the Red Arm, and the terrible warrior Liath Luachra (Grey Rushes), a chief disgraced by Cumhail. Coil was promised the command of the Fianna, and Liath Luachra the magic Corrbolg (Body Defence) of Cumhail, and the fisherman of the Boyne, who was accustomed to take in three draughts at the mouth of that "yellow-valed, ever-beautiful river, as many fishes as sufficed for a meal to all the forces of Cumhail."
So defiance was made, and a pitched battle appointed on the grassy plain on the east side of Cnucha (Castle Knoc). While the preparations, were making, Cumhail met in one of his hunting excursions the fair Muirrean, daughter of the powerful Druid, "Tadg of the Luminous Side," whose abode was the fortress of Almuin (Hill of Allen). Evil fate at the time gathering thick round the path of the warrior, he forgot his knightly vows, and that reverence for female purity so deeply inherent in the Celtic disposition.
Deep and torturing remorse seized on him when too late; but her incensed and powerful father vowed by his gods to devote him to irrevocable defeat and death. When the day of strife approached, he sent the swift-footed Balar to the fairy hill of Maev the Sighe-Queen, who abode in the enchanted hill at Carmain (Wexford), for the impenetrable coat of mail the Corrbolg, and the accompanying resistless, jewel-hilted glaive and spear. But the revengeful Tadg spread such a thick druidic fog on his path, that he was unable to find the enchanted mound. So Cumhail was obliged to content himself with the inferior arms furnished by Aoiné (Venus), the presiding Sighe-Queen of Naas.
Warnings were given to Cumhail's confederates that for his forgetfulness of his vow, which in common with all the Fianna he had made, never to do wrong to woman, defeat and death awaited him at Cnucha. Most of them had scattered homewards before the day of battle dawned. With a strong presentiment of his ill-fortune full upon him, on the eve of the battle he called before him Boghmin, his female runner, and thus addressed her:--
"My fleet and faithful Boghmin, the night of the grave is gathering fast round me, and I address you for the last time. Speed to the rath at Almuin. Seek the golden-haired Muirrean, and tell her my chief sorrow on quitting life arises from the wrong she has suffered at my hands. Attend her diligently, and when my son is born flee away with him, and let him be brought up in the most secret places you can find. Otherwise the wrathful Tadg will destroy him. The wise Conmean, the Druid, has foretold his fortune, and that under his rule the fame of the Fianna of Erinn shall much exceed what it enjoys under mine. Entreat the forgiveness of the golden-haired Muirrean for me. Farewell!"
When the next morning sun was shining pleasantly over the gorse-sprinkled plain (now the western portion of Phoenix Park), the clans of Leinster, and those of the noble Eogan of Munster, a small but compact force, rushed among the, multitudinous hosts of Ulster, Conacht, and Meath, even as a score of noble and fierce dogs would among a pack of desperate and ravenous wolves. They overturned them, they pierced their masses; they levelled, and slew, and scattered their foes, who were still replaced by fresh and strong fighting men, till their tired arms could scarce wield the sword or fling the spear. Hundreds and hundreds perished that day by the resistless arms of Cumhail; and when he and Goll, son of Morna, engaged, it was like the meeting of two hawks on a rock, two furious bulls, or two raging lions. Deep was the hard brown skin of Goll gashed by the keen blade of Cumhail; but when they were separated by the press of struggling warriors, the heavy ash-shafted and keen pointed spear of the Conacht hero darted with mighty force, tore the lorica, the silken shirt, and the heart of the mighty son of Trenmor asunder, and the green sod was reddened with his life's blood.
Again and again did the ear-piercing war-bugle of Eogan gather the Leinster and Munster troops to the waning fight; and when all hope was lost they retired in close array, and still kept their furious foemen in check. Thus ended the memorable fight of Cnucha, in the account of which no mention is made of sling, arrow, bow, mace, or battle-axe. The bronze leaf-shaped, double-edged glaive, and the spears and javelins flung over hand, were the same as those sung by Homer as' doing the work of death on the plain before Troy.