On the underside of blue plastic barrel used as a racing mark in Strangford Lough.
No interactions present.
without a scale for size (millimetres count) it is hard to judge.
....but the other barnacles that look like this
B.improvisus - usually brackish/estuarine waters and the sutures meet differently to the pic
B.perforatus - much more symmetrical and very big when older.
The pics show mature barnacles - acorn barnacle crenatus for me too.
Lat/Lng: 54.39, -5.621
OS grid ref: NW650054