In the village of Lavally about three miles from Clarenbridge there lies a Cromlech. The field where this Cromlech lies belongs to Thomas Heaney now but it was sold to him a few years ago by William Feeney. The Cromlech lies within 20 yds of the road wall. This field belonged to William Feeney when the Cromlech fell about eight years ago.
It is thought that this Cromlech would not have fallen only for a constant grazing of the sheep wore the clay from around the three standing stones so that they could not afford to hold up the top stone. The top stone is about 26 1/2 feet in circumference and about a foot and a half in height. The perimeter of the underneath is about 14'2".
It is not known for certainty why this Cromlech was erected. Most people think that this was a druid altar in olden times but it is also thought that Diarmuid and Grainne took shelter under this Cromlech. There is also another stone in the wall not far from the Cromlech and is exactly like the stones of the Cromlech. It is thought that this stone was brought to be put in the Cromlech but it was brought no further.
It's not looking so good at Lavally, as you can see by this 2002 photo on FourWinds' Megalithomania site.
The illustration from Borlase's 'The Dolmens of Ireland' book (made towards the end of the 19th century) shows rocks on top of the dolmen. This isn't mentioned in the text. But it rings a bell with me... I'm sure I've read about people somewhere throwing stones up onto cromlechs for luck, or maybe for a more specific outcome. I'll have to look into it. Maybe people still do it elsewhere...