Canmap describes two stones here. They're only about four feet high and are both 'natural stones with no artificial markings'. This folklore admittedly doesn't involve them, but is about their environment above the steep slopes down into the city.
The Glasgow chronicle (January, 1826) records the following occurrence at Paisley, on the occasion of some silkworkers being out of employment.*
"Visions have been seen of carts, caravans, and coaches going up Gleniffer braes without horses, or with horses without heads. Not many nights ago, mourning coaches, too, were seen going up the Cart above the town, with all the solemnity of a funeral. Some hoary-headed citizens relate, that about thirty years backward in their history, a famine was prognosticated in much the same way, by unusual appearances in the Causey-side. The most formidable witnesses in favour of the visions come from Neilston, who declare that they have seen the coaches, &c. two by two, coming over the braes, and are quite willing to depose to said facts whenever asked, before the Paisley magistrates."
They're probably visible at the moment, as I assume this bit [*] refers to Woeful Economic Climate.