The surrounding district is limestone country, yet this conglomerate block, roughly 1.7 metres long and 0.75 metres high, contains porphyritic and quartzite inclusions that have no business in this landscape. Someone, at some point, brought it here deliberately. On its upper surface are two basins hollowed into the rock; a bullaun stone is a boulder or outcrop bearing one or more such cup-shaped depressions, often associated in Ireland with early ecclesiastical or ceremonial sites. The larger of the two basins at Toonagh is clearly artificial, oval in shape and approximately 0.55 metres across, 0.36 metres wide, and 0.1 metres deep. The 1921 Ordnance Survey six-inch map records it simply as "Basin Stone", a name that acknowledges the visible feature while saying nothing about its purpose.

What makes the stone stranger still is its proximity to an inauguration mound, sitting just 31 metres to the south. Inauguration mounds were raised earthworks used in Gaelic Ireland for the formal installation of kings and lords, and the landscape around Toonagh retains traces of such a site. The stone is aligned east to west, which may or may not be significant. Whether the bullaun played any role in inauguration rituals is genuinely unknown. Elizabeth FitzPatrick, writing in 2004, noted a parallel worth considering: a basin stone also occurs at Dunadd in Argyll, the early medieval fortress and inauguration site of the kings of Dál Riata in what is now Scotland. That comparison raises the possibility of a shared ceremonial tradition across the Irish Sea, without resolving what either stone was actually used for. The honest answer, for Toonagh as for many such monuments, is that the function remains unclear.