The name Meini Gwyr should have a to-bach (or circumflex) over the ‘y’ for it to be read corectly in the Welsh. The word maeni means stones, and the word gwyr means either men or husbands.
If the to-bach was over the ‘w’ then the word would becomes crooked, which makes a bit more sense to me. Having said that, I’ve never seen this spelling, but on Stukley’s illustration there is no to-bach at all. This suggests that the name may originate from either ‘men/husbands’ or ‘crooked’.