Robert Burns' poem 'Tam o'Shanter' is about a bloke who gets repeatedly boozed up when he goes to the market. And on his way home late one night, he passes a church and looks in, and all sorts of dreadful stuff is going on inside, and the devil accompanying it all on the bagpipes. But stupidly he draws attention to himself, and has to gallop off to escape. His horse is just on the bridge and they're nearly safe - because if you can get across the middle of a stream of running water, you're ok - but the witch at the front reaches out her hand, and his poor horse loses her tail. http://www.robertburns.org.uk/Assets/Poems_Songs/tamoshanter.htm
Well that's the poem. But this hill is where it Really Happened, honest.
The man involved was an Adam Forester, or maybe Foster, and he didn't shake the witches so easily - although he got across a stream they used a bridge downstream (these witches could use bridges, which seems reasonable) and caught up with him on Waterside Hill.
Finding that neither he nor his horse could get a foot farther, the determined fugitive alighted, and unsheathing a sword, on whose blade was engraven the sacred name of Jehovah, he twice waved it around him, and then describing a circle with its point on the sward, he charged, in the name of God, his pursuers not to overstep that circle. The mysterious band - furious as they were - stood, as mysteriously arrested. They had no power to overpass the circle; - but surrounding it, menaced the horseman until a neighbouring cock crew, when one of the most inveterate of the gang drew a large knife from beneath her apron, cut the horse's tail, which, it seems, hung beyond the verge of the sacred circle. They then scampered off; and the horseman, standing firm in the ring with the drawn sword still in his hand, awaited the day-break, and then, renewing the circle, and giving thanks to his Maker, he rode home to his residence.
[...] Certain it is, that Foster, as long as he lived, and his sons and grandsons after him, made a point of renewing the circle annually.
From 'Unique Traditions Chiefly of the West and South of Scotland' by J G Barbour (1886).
But I'd like to bet that the circle on Waterside Hill is really the cairn. But someone will have to go and have a look to answer that.