The inscription on the stone reads RIALOBRANI CUNOVALI FILI.
I have read that as this is not proper Latin, but a version being used at the time of carving by locals living on the edge of the Roman Empire, an exact translation is difficult, but “of the Royal Raven, son of the Glorious Prince” is close.
Source: Antiquities of West Cornwall. Guide 1: The Men-An-Tol Holed Stone
[ISBN 0-9512371-2-8]
menantolstudio.freeserve.co.uk/web%20page%203(2).htm