The reason why I say that this rock does not appear to be prehistoric are two fold.

Firstly, the local rock type is hard volcanic rock. Just check out the Copt Howe carvings and the difficulty the carvers had in executing the motifs (depth & precision). To carve such a beautiful bowl would have been extremely difficult.

Secondly, looking at the shape of the bowl & it's run-off, I would guess that this stone was carved in situ i.e. at an angle, again extremely difficult given the nature of the rock, the precision of the carving and the tools available to the prehistoric masons.

I'm not denying the existance of bullauns in Britain, we have many prehistoric examples ranging from big cups to rock-cut bowls (it is regretful that we don't have an english word that has the poetry of 'bullaun') but in this particular case I would guess that, yes it may be a bullaun, but that it is not prehistoric in origin.