Cystoseira baccata is a type of brown seaweed in the family Fucaceae. The name baccata means “berry-like” and refers to the small air bladders.
It is commonest in France and Spain, and in Ireland and the UK where it is mostly found in the Channel Islands, the south coast of England and the west coast of Ireland. However in this case I found it in County Down. It is found mostly on the lower shore and the sublittoral zone attached to rocks or higher up the shore in deep pools with sandy bases. It was first described and illustrated by Gmelin who wrote Historia Fucorum (1768)
Cystoseira baccata – https://en.wikipedia.org/wiki/Cystoseira_baccata