Known locally as O'Brien's Rock, it marks a site where the O'Breen chieftains, also recorded as O'Bryan or O'Brien, were formally inaugurated as lords of their territory. Inauguration sites of this kind were a distinctly Irish institution; a chieftain would be installed upon a sacred stone in a public ceremony that conferred legitimacy and announced continuity of rule. That such a site survives here, unremarked in ordinary farmland beside one of Ireland's great rivers, says something about how thoroughly the landscape can absorb its own history.

A description from 1973 records the rock as it then appeared: large, roughly circular in shape, with a flat top, sitting quietly on the Shannon's eastern bank. Earlier still, a 1945 account by English identified it as the inauguration place of the local O'Breen chieftains, connecting the physical stone to a specific Gaelic dynasty and their ceremonial life. The place name itself carries that association forward; Carrickobreen derives from the Irish for the rock of the O'Breens. A related enclosure lies close to the south-east, suggesting the inauguration stone did not stand entirely alone in the older organisation of this landscape.