On the bank of the river near Slidry, there is a long grave-like mound, distinguished by two large erect stones standing, the one at the head, the other at the foot, at an intervening distance of about 30 feet. This is supposed to be an elongated trench, in which the warriors slain in battle have been buried; tradition claims it as the grave of one of Fingal's heroes.
from 'On the Rude Unsculptured Monoliths and Ancient fortifications of the Island of Arran' by Mr John McArthur. In 'The Edinburgh New Philosophical Journal' - v9, 1859.
On the bank of the Slidry stream, to the south of Arran, there is an elongated, ship-like cairn, exactly similar to the celebrated currach mound of Iona. It is thirty feet in length, with a smaller ridge attached, measuring nine feet. The sides of the tumuli are trenched with flat, flag-like stones, and at each end there stands a large monolith of red sandstone, encrusted with lichen and moss.
This monument is supposed to mark the grave of one of Fion-gal's heroes, about whom many strange stories are told. An anxious treasure-seeker who dug into the larger mound, is said to have found a huge bone, into the hollow of which he thrust down his foot and leg as into a boot.*
*Headrick's Arran, p148.
Headrick was writing in 1807: a book called 'View of the Mineralogy, Agriculture, Manufactures and Fisheries of the Isle of Arran'.
It sounds like the cairn suffered a lot in the 19th century. The RCAHMS record suggests remains still exist (though the name "couldn't be confirmed locally" in 1977).