Natural History Museum curator, Jan Freedman replies:
The picture looks like fossil coral in limestone. Often they are weathered at strange angles, so the view you see is an irregular cross section, and so it doesn’t look like a coral. Fossil coral links to the type of rocks around Donegal Bay, as they are Carboniferous in age and there are Carboniferous limestones with corals in them. It is probably a Lithostrotion coral but I'm afraid it's hard to tell which species.