Taken from the Irish Folklore Commission 1937/1938 Tipperary Reel 4 pg 207:
The longstone according to this was originally part of a pair. It relates to the Diarmuid and Grainne story. Diarmuid carried this one on his back (The longstone) and Grainne carried a smaller one. Which is now gone.
Seemingly there is the print of a chain at the bottom of the remaining longstone. This was where Diarmuid used a chain to strap it on his back while carrying it.