Dolmens.-- These were supposed to be giant’s graves, and, if called “altars,” the word was understood in a Christian sense, with a belief that they had been used for the mass during the prevalence of the cruel penal laws.
For example, Altoir Ultach was said to be named from an Ulster priest who served the mass there in the eighteenth century because the nearest magistrates were more tolerant than those of the north.
There is no evidence of any general popular belief that they were pagan altars, such an idea, where it existed, being derived from the “learned ignorance” of local gentry.
p91 in
A Folklore Survey of County Clare (Continued)
Thos. J. Westropp
Folklore, Vol. 23, No. 1. (Mar., 1912), pp. 88-94.