The original purpose of bullan stones is not really known, but they have an undisputable association with water and Brigid worship. A 'bullaun' is a deep hemispherical cup hollowed out of a rock. Bullaun Stone refers to the rock itself, which can have many bullauns in it, although many are single.
It is generally thought that they date from the Bronze Age, but I personally believe there is a much old provenance to them and that there is a relationship to prehistoric rock art, for a good example of this see Glassamucky Mountain (County Dublin).
Ritual use of some bullaun stones has continued well into the Christian period and many are found in association with early churches ( The Deer Stone (Glendalough D) (County Wicklow) is just one of many at Glendalough (County Wicklow)) and holy wells. Their presence at so many early Christian sites, to me, places them as being of massive importance to the pre-Christian inhabitants of Ireland and something the church was very eager to assimilate.
The beautiful example at St Brigit's Stone (County Cavan) still has its 'cure' or 'curse' stones. These would be used to by a visitor turning them whilst praying for (or cursing) someboby.
