Have to admit to sharing your mystification over what bullauns are and yes, this one, called a bullaun on archaeology.ie, is very suspicious. Were it a millstone, would the hole go right through? This doesn't. Also, the stone is an earthfast boulder, more square than round. A cross-base? Maybe, maybe not. The tall stone in this shot is a cross-base. Scalp looks more like this, a hole carved by army sappers to erect a flagpole.
So no, I'm not sure. Then again, I'm not sure why I bother trudging through muddy fields hunting out stones in the first place! :-)