Without a Welsh-English dictionary to hand I had a go at translating the name Cylch Derwyddol and came up with ‘circle of the oak meadow’. I based this on the assumption that ‘derwyddol’ was derived from the words ‘derw’ meaning ‘oak’, and ‘ddol’ meaning ‘meadow’. [Judging by Tollvên’s post, I got this entirely wrong].
Cylch Derwyddol lies on the eastern slopes of a mountain called Moel y Garn, which translates as ‘bare or rounded mountain of the cairn’.