Rockweed
Rockweed (Ascophyllum nodosum); also known as Knotted Wrack
Photo at Tin Can Beach: Adam Chateauvert (Feb 28, 2014)
Here is a great video of Ascophyllum nodosum from Michael D. Guiry of the AlgaeBase programme, National University of Ireland.
Knotted Wrack (Ascophyllum nodosum), also known as Rockweed, is a common intertidal seaweed in Ireland, particularly in sheltered places. When the tide is in, it floats to the surface to get as much light as possible (and probably to shade other, competing weeds out.
Bladderwrack
Bladderwrack (Fucus vesiculosus)
Photo at Tin Can Beach: Adam Chateauvert (Feb 28, 2014)