The cairn on top of Carn Mor is modern, but it's thought to hide the remains of a prehistoric version. There are a few boulders around the edges which could have been kerb stones.
Grimm refers to a remarkable instance of this superstition, which occurred in the island of Mull as recently as 1767, which vividly illustrates the "toughness" of tradition, as Dasent expresses it. He says:- "In consequence of a disease amongst the black cattle, the people agreed to perform an incantation, though they esteemed it a wicked thing. They carried to the top of Carnmoor a wheel and nine spindles, long enough to produce fire by friction. If the fire were not produced before noon, the incantation lost its effect.
They failed for several days running. They attributed this failure to the obstinacy of one householder, who would not let his fires be put out for what he considered so wrong a purpose. However, by bribing his servants, they contrived to have them extinguished, and on that morning raised their fire.
They then sacrificed a heifer, cutting in pieces and burning, while yet alive, the diseased part. They then lighted their own hearths from the pile, and ended by feasting on the remains. Words of incantation were repeated by an old man from Morven, who came as the master of the ceremonies, and who continued speaking all the time the fire was being raised. This man was living a beggar at Bellochroy. Asked to repeat the spell, he said the sin of repeating it once had brought him to beggary, and that he dared not say those words again."
I don't know where he's quoting this Grimm from, but this is collected in Hardwick's "Traditions, Superstitions, and Folk-lore" (1872).