Well taking my clue from Garmon,(a saint) came up with this...
the story goes thus;Local traditions say that Garmon came to a particular area in 447 and sought refuge at mountain stronghold of Benlli Gawr, a giant chieftan but was refused admission, however Cadell took pity on him and gave him land at Llanarmon yn Ial and Llanarmon Dyfyrryn Ceirog.. so that gives the circular churchyard some early iron age history.
St.Garmon's history, churches and chapels are to be found all over Wales, some by standing stones and cromlechs, so there is good reason to suppose something would have been in the churchyard for it to be dominated by the celtic church..
Book of Welsh Saints - T.D.Breverton.