To be honest the one in fitz's photo doesn't look like I'd class it as a bullaun - the 'cup' isn't smooth enough.
Many that are used in churches in Ireland are most probably much older, but some are going to be originals. I tend to go on the proximity of a stream to the site to get a clue as to whether it's older or not.
The small ones (about the size of the one fitz linked to) tend to be new if they're in churches. If they're in bedrock then they're almost certainly old.
Not much help is it?