Tim
The line in my version goes:
In Kilkenny, it is reported/There are marble stones/as black as ink.........
Marble stones, as far as I am aware were smooth stones used for beating the water out of clothes before drying them. Being Kilkenny, these rocks were probably basalt, hence their being black. Just why such a thing should be featured in the Carrickfergus song beats me, but remember the singer is thinking about his home while in a drunken haze - ("I'm drunk today/And I'm seldom sober") so maybe that's why!
Cheers!
Graeme