6O'Donovan, John, and Michael O'Flanagan. Letters Containing Information Relative to the Antiquities of the County of Donegal, Collected during the Progress of the Ordnance Survey in 1835. Vol. 5. Bray, 1927. 53+.
O'Donovan was told by an old O'Donnell gentleman that the inauguration stone in the Kilmacrennan church was "destroyed by a Mr. Mac Swine, who having changed his religion, became a violent hater of everything Irish. He tore down a great part of the old Church to obtain building materials and destroyed all the ornamented stones in the neighbourhood."
Another author suggests that the most significant medieval O'Donnell inauguration site was not The Rock of Doon but rather was at Carraig an Dunain, close to Donegal town. (Ó Canann, Tomás G. "Carraig an Dúnáin: Probable Ua Canannáin Inauguration Site." The Journal of the Royal Society of Antiquaries of Ireland 133 (2003): 42.)
Elizabeth FitzPatrick notes that traditions of the Rock of Doon's use for royal ritual are "based on complex local folktales recorded and reiterated from the beginning of the nineteenth century onwards." ( (FitzPatrick, Elizabeth. Royal Inauguration in Gaelic Ireland C. 1100-1600: A Cultural Landscape Study. Woodbridge, Suffolk, UK: Boydell, 2004. 184.)
More details of the inauguration rite may be found here.