According to the Bodleian
Dinnshenchas, here are two theories of how the mountains were given their name: [11. SLIAB BLADMA.] Bladma or Blod, son of Cú, son of Cass Clothmín, killed the cowherd of Bregmael, the smith of Cuirche, son of Snithe, King of Húi Fuatta. Then he went in his little boat till he set up at Ross Bladma — Ross n-Áir, "Wood of Slaughter," was its name at first. Thence he went to the mountain. Hence is "Sliab Bladma" (Bladma’s Mountain). Whence the poet said: ‘Blod, son of Cú, son of Cass Clothmín, Killed the cowherd of fair Bregmael, The smith of Cuirche Mór, son of Snithe: He set up at Ross Tíre ind Áir.’ Or it is Blod, son of Breogan, that died there; and from him the mountain of Bladma was named.
Edward J. Gwynn's The Metrical Dindshenchas give a longer account and another origin story (
bleda mara "sea-monsters" cf. Modern Irish
bleidhmhíol "monster; whale"):
Sliab Bladma Blod, son of Cu, son of Cass the renowned, son of Uachall the many-shaped, killed Bregmael the famous smith of Cuirche, son of Snithe the swimmer. Curche Cendmar was a daring king over Medraige and over Herot; through him Blod, son of Cass Clothmin, found never sure protection. He fared in his ship–clear purpose from the Bottom of pure-cold Galway, from Ath Cliath in wide Herot to Ath Cliath in Cualu. Thence he came after many a turn to the Point of Nar, son of Edliuc, and possessed, as his special portion, the mountain whose name derives from Blod. A valiant man who used to wage battle died at Sliab Bladma–vast renown even Blad, son of Bregon, with troops of warriors, died of disease in the monster-haunted Sliab Blod. Or, it is from the son of Bregon the wrathful that it is named Sliab Bladma, with onsets of women their increase is not far from the cattle was the mountain where it happened through strong Blad. Or the monsters of the sea that was not calm, beasts–
ruisenda was their name–came throughout the land of the tribes, so that from them is named Sliab Bled. Blod, son of Cu, son of Cass Clothmin, slew the herd of Bregmael the smith of Curche, son of Snithe, he settled at Ross Tire Nair. The Modern Irish meaning of
bladhm is "flame; flare up",
bladhma being the
genitive case. ==History and mythology==