There are actually two bullaun stones at this church, one either side of the path. The one is not really worth significant wordage and has one shallow cup and a broken one. The other is quite exceptional! it has two small (8cm) cups and one massive (25cm) cup.
This larger cup truly is a marvel and well worth a visit.
From Ardglass take the A2 coastal road north for about 3km until you reach Chapeltown. Near a very nasty junction you will see a church on the left. The bullaun stones are either side of the path leading to the church.
This is an explanation of (and a bit of a disclaimer for) the coordinates I provide.
Where a GPS figure is given this is the master for all other coordinates. According to my Garmin these are quite accurate.
Where there is no GPS figure the 6 figure grid reference is master for the others. This may not be very accurate as it could have come from the OS maps and could have been read by eye. Consequently, all other cordinates are going to have inaccuracies.
The calculation of Longitude and Latitude uses an algorithm that is not 100% accurate. The long/lat figures are used as a basis for calculating the UTM & ITM coordinates. Consequently, UTM & ITM coordinates are slightly out.
UTM is a global coordinate system - Universal Transverse Mercator - that is at the core of the GPS system.
ITM is the new coordinate system - Irish Transverse Mercator - that is more accurate and more GPS friendly than the Irish Grid Reference system. This will be used on the next generation of Irish OS maps.