On the summit of Brewel Hill in County Kildare, at around 221 metres above sea level, a large circular earthwork encloses something stranger still: a cluster of mismatched boulders, two of granite, one of white quartz, and one of red conglomerate, known locally as the Piper's Stones. The quartz boulder, which occupies a roughly central position among the group, has long been called the Piper's Chair, a name recorded as far back as the late nineteenth century on account of its vaguely seat-like form. What makes the whole arrangement unusual is not just the stones themselves but the layered geometry around them. The monument is not a simple ring but a nested one: a large outer bank and fosse, roughly 76 metres in external diameter, enclosing a second, smaller embankment of around 26 metres, which in turn contains the stone grouping. The inner enclosure sits eccentrically, off-centre relative to the outer ring, which gives the entire complex an asymmetry that resists easy classification.

FitzGerald noted the earthwork in the 1890s, describing a wide double entrenchment already much levelled by that point. By 1931, Walshe was recording individual stone dimensions: the large granite boulder standing around five feet high, the oblong red pudding stone measuring five feet three inches in length, and an incised hole visible in the larger granite. A 1972 survey found that the southern half of the monument had been planted with spruce trees, and the four boulders visible at that time were arranged in a semicircle, prompting the suggestion that corresponding stones in the northern half had been removed or never existed. Geraldine Stout, examining the site in 1980 for her MA thesis, identified the outer ditch as possibly causewayed at the north-north-east and noted that the main circular ditch was unusually wide, around 15 metres. Crucially, she observed that the presence of an external fosse, rather than the internal one typical of most Irish earthworks, connects this monument tentatively to the henge tradition, a category of prehistoric ceremonial enclosure more commonly associated with Britain but not unknown in Ireland. A possible cursus, a long parallel-ditched monument also associated with prehistoric ritual, lies to the east-south-east, and four ring-barrows are situated downslope to the north-east, suggesting that Brewel Hill formed part of a wider ceremonial landscape in prehistory. A coniferous plantation was at some point established across part of the monument's interior, a fact that continues to complicate survey work and access to the stones.