There's a perforated standing stone in a churchyard close to the end of the Lleyn peninsular that fits your description. Sadly, that description is all there is. I think it's shown on the OS.

Small Celtic note; There is a ritual votive offering site in the Llyn Cerrig Bach lake (probably near Holyhead, Valley is the village, but can't confirm) 170 pieces of top quality stuff, chariots bits and pieces, slave chains, animal bone including horses(sacrificial), crescent shaped plaque. High status metalwork deliberately thrown into lake.The dig was done by Cyril Fox in 1946.
She also mentions the Goddess Brigantia, who stems from the Irish Goddess Brigid. Her ref;
"She is invoked as Dea Nymphae Brigantia, a pointer in her connection to water.. she goes on to say that the rivers Braint and Brent(earlier Briganti - Middx) have a strong adjectival significance. Presumably Braint is a river in Anglesey.. She links the deposit to a 61 ad date, a final desperate act in the face of the Roman invasion.

Ayup!

Did you mean TMA the book rater than this site - coz 4Ws & others are right (IMO) in saying you won't find much that ain't on ere.

Or did you want recommendations from the stuff that ain't in the book?

love

Moth