In the vicinity of the village of Dunlop, writes Chalmers in 1824, "there was in former times a chapel dedicated to the Virgin Mary [..] After the Reformation, this chapel was allowed to fall in ruins, but the remains of it are still to be seen on the side of a small rivulet which was here crossed by stepping-stones called the Lady's Steps, and this name is still continued altho' the steps have been superseded by a bridge." (Caledonia, vol. iii. p 556.)
[..] In a field in the neighbourhood is a large detached stone, round which, if tradition is to be believed, it was customary for persons attending the chapel to perform part of their devotions. It is called the Thugart Stane, supposed to be a corruption of the grid stane. This stone, the name of which is by the inhabitants of Dunlop commonly pronounced "Ogirtsane," is composed of a variety of trap rock, differing from the trap formation in the surrounding country. What appears of it above the surface measures about 12 feet by 8, and its greatest height is about 4 feet.
Coflein is determined that this is a natural stone, which it surely is. It's a big and noticeable one though, at 13ft 2ins by 10ft and 4ft 1in high, "and is a natural erratic, of blue augitic porphyrite." But its presence has been linked in local consciousness with the once-present St Mary's Chapel (of which no trace is now said to be), St Mary's well, and the tradition of a burial site on the hill. The Coflein record also mentions rumours "that it was a 'Druidical Alter', or used for performing acts of mortification in connection with worship at the nearby chapel". It's interesting that the following quote mentions Special Protection afforded to the stone.
At a place called the Chapel Craigs, about half a mile from the village of Dunlop, there existed until lately the ruins of a chapel, which was dedicated to the Virgin Mary... It stood upon a rock, on the side of a rivulet, which was crossed by steps, called the lady's steps - which steps, however, have been superseded by a bridge. A beautiful stream of water gushes from the rock.
The existence of this chapel has given name to a number of localities around. A few hundred yards south-west of the site of the chapel, on the gentle swell of the hill, is a Druidical stone, called the Thugart stane, supposed to be a corruption of the grit stane. It appears at one time or another to have been a rocking -stone. The base is so covered with rubbisth, that it has now lost its vibratory motion. It lies on the farm of Brandleside, and the tenant is bound in his tack to protect it, by neither removing it, nor cultivating the ground for a considerable number of square yards around it.
Above the site of the chapel, a pathway was cut out of the solid rock, leading to the top of the hill, where tradition says there was a burying-place belonging to the chapel. The pathway is nearly obliterated, a quarry having been opened in the place a number of years ago.
From 'History of the County of Ayr' by James Paterson (1852) p45.