George: this from Flanagan's "Irish Place Names."

CARRICKFERGUS, Antrim. "Carraig Fhearghasa - rock of Fergus".
"Cnoc means anything from a hill to a small mountain".

In Carrickfergus meaning, it may signify instead "fortress, castle": in the sense of solid, strong, permanent as a rock. (My opinion, not "official" from Flanagan)

Kilkenny black marble was also used for other purposes, notably in the construction of fireplaces. The Behan family had one, and it is interesting that Brendan's biographer, Michael O'Sullivan (1997), says that Brendan and Dominic learned "Carrickfergus" from the singing of their mother, Kathleen. This was long before Peter O'Toole came on the scene. Personally, I suspect the O'Toole story is a tall one.