With my almost non-existent geological knowledge I’d have called this an island. Some web searching reveals that it’s really a reef - “a ridge of jagged rock, coral, or sand just above or below the surface of the sea”.
Whatever the term, the exposed part has a generous coating of grass and white flowers.
Photographed, from close to this car park J6174 : Car park, Ballyhaskin, Millisle (August 2017), about 110 minutes after high water Donaghadee.
I was born a couple of miles away from where that picture was taken - it is in fact Millisle Borstal, formerly the home of Pac Berisford - now diseased. Ballyrolly as far as I recall was a lane down by the Knowse.