This little area near Foel and Garthbeibio has / had quite a few stoney and watery things of interest.
The cairn is where the Afon Banwy and the Afon Twrch converge. I'm guessing it's the one mentioned here. A couple of pages on another cairn is mentioned (Cae'r Dentyr Cairn) which was very nearby, and the stones from that were taken for the nearby bridge.
Also close by was an immense stone, 'Y Maen Llwyd' (the story is told at Llymystyn Camp, where it was apparently thrown from), but this was regrettably broken up in the early 19th century to help make a wall.
There are three wells: Ffynnon Ddu (on the lane on the way to St Tydecho's church); Ffynnon Rhigos ('formerly of repute "in healing the eyes." The water, sweetened with sugar, used to be drunk by the parishioners upon certain feasts'); and St Tydecho's Well itself: 'This is now filled up, and the water diverted to a drain which runs down to the high road below St. Tydecho's church. "There was once an image of the saint's head, in stone, placed at the northern side of the well; but some vandals, having no regard for remains of antiquity, nor even respect for common decency, threw it away; and the last heard of it was a plaything on the side of the river among some children, who, in the end, threw it in, and no more was heard of it" (Mont. Coll. 1873, vi, 13). Parishioners yet survive who remember persons coming to bathe in this well, which was of reputed efficacy for the cure of rheumatism.'
Standing stone, celtic christianity, holy water, cairns, a confluence, a disembodied head? It's tempting to put all these romantic notions together and conclude this was quite a special location. (and maybe make 2+2=5 of course).