This is a Dark Age christian memorial stone from around the 6th century CE. The RCAHMW gives the inscription as ‘CUNOGUSI HIC IACIT’ [Latin for ‘Cunogusus lies here’]. The letters are still fairly easy to make out.
There is a rumour that the stone has a cup mark on one side, making it a re-used prehistoric menhir, but I couldn’t find it.
Apparently the name Cunogusus is preserved in the name of the nearest village, Pencarnisiog, which if true would be a wonderful example of continuity over the last twelve centuries.