12Harbison, Peter. Pilgrimage in Ireland: the Monuments and the People. Syracuse, NY: Syracuse Univ., 1992. 86.
The author (quoting William Wakeman) offers that the word derives from the Irish word bullaun meaning "little pool." But the Sites and Monuments Records definition of a bullaun is: "...(from the Irish word Bullá, which means a round hollow in a stone, or a bowl) is applied to boulders of stone with artificially carved, hemispherical hollows or basin-like depressions, which may have functioned as mortars. They are frequently associated with ecclesiastical sites and holy wells and so may have been used for religious purposes. They date to the early medieval period (5th-12th centuries AD)." (Kelleher, Matthew, and Caimin O'Brien. "Between a Rock and a Hard Place." Archaeology Ireland 22.3 (Autumn, 2008): 8-9.)
An 1846 text reports a "respectable farmer" declaring "that he was not above saying a prayer at the 'blessed stone' when he came that way," and noted that "the water found in hollows of bullan stones was held good for bad eyes." (Bonwick, James. Irish Druids and Old Irish Religions. London: Griffith, Farran & Co., 1894. 80-81.)