OK ... here goes ...

Bullauns are essentially very large cups/basins. In Ireland anything over 5cm in diameter counts as a bullaun and they reach 30cm+.

They are usually hemi-spherical, although there are some flat botomed ones and some conical ones in county Carlow.

They are usually associated with water, usually running water either by rivers or springs. They can be found on lake shores though or by wells. One common link seems to be Brigid or these days St. Brigit.

The accepted date for many is Iron Age to early Christian. I personally believe that many are Neolithic.

Purpose? Nobody really knows. Try some of these:

Domestic querns. Ritual querns. Scrying pools. Fonts. Things to hold the base of a tree trunk for some purpose. Pestles for preparing medicinal herbs. A few others I can't think of right now.

I love them. They're so enigmatic and touchy-feely. There's something oxymoronically organic in their man-made-ness if you know what I mean. For some reason I also think of them as being very feminine - I've always found that idea to be very weird!

Can't add much to what FWs said 'cept to say I understand your scepticism, if all you've seen is the Deer Stone. Here's somehting to ponder: http://www.themodernantiquarian.com/post/52527
I don't know when this was done, or by whom, but I'm glad they did it. Of all the stones I've seen in my relatively short time at this I can say that this is a thing of true beauty (which is not to say that other stones aren't beautiful). Bullauns (from the Irish for bowl I believe) have this otherness about them, a mysterious depth that tugs at a part of me that I don't have a name for. Besides, they look cool and they make me smile and, the best bit, nobody knows for sure what they were for so you can come up with your own idea.