I can't find a story for the hill of Knockdolian itself, but I was here in the summer, and it's the most stupendous landmark, looking just like the nearby giant limpet-shaped island of Ailsa Craig from some angles. I liked the hill a lot and I imagine the views from the top would be marvellous. It's topped by a 'grass covered cairn .. composed of large and small stones, with rock outcrop protruding in places.. 2m high.. a few large kerb stones are visible', according to the info on Coflein. But here's some local stoney folklore:
An old family once lived in a house called Knockdolion, which stood on the banks of the Water of Girvan in Ayrshire. There was a black stone at the end of the house, and a mermaid used to come and sit on it, combing her hair and singing for hours on end. The lady of the house could not get her baby to sleep because of the loud singing of the mermaid, so she told her men-servants to break up the stone. This they did, and when the mermaid came on the night that followed she found no stone to sit upon. She at once flew into a rage, and cried to the lady of the house:-
Ye may think on your cradle-
I think on my stane;
There will ne'er be an heir
To Knockdolian again.
Not long after this the baby died. He was the only child in the house and when his father and mother died the family became extinct.
A harsh punishment but you mustn't go messing with stones.
From 'Wonder Tales from Scottish Myth and Legend' by Donald Alexander Mackenzie (1917).