Twelve miles from land, in the German Sea. It is dangerous for navigators, and therefore the abbot of Aberbrothok fixed a bell on a float, which gave notice to sailors of its whereabouts. Ralph the Rover, a sea pirate, cut the bell from the float, and was wrecked on his return home on the very rock. Southey has a ballad on the subject.
Precisely the same tale is told of St. Goven's bell, in Pembrokeshire. In the chapel was a silver bell, which was stolen one summer evening by pirates, but no sooner had the boat put to sea than all the crew was wrecked. The silver bell was carried by sea-nymphs to the brink of a well, and whenever the stone of that well is struck the bell is heard to moan.
N.B. Inch or Innis means island.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Inchcape Rock from Infoplease: