Hey Victor,

Did you check the plausibility of this archaeological site with other sources?

It does look circular at first (on Bing).
(ie. think of a large enclosure)
Zooming in it looks fairly regular, octagonal (or even square) in shape.
(ie. still enclosure, or moat)
East edge is not visible, but that is okay.
The west edge seems more convincingly circular when viewed in Esri Clarity, but only as thin edge.
First edition maps have a regular boundary along the west edge.
Looking at aerial imagery the area is wet, there is some streams and tracks, no clear ditch.
https://livingatlas.arcgis.com/wayback/#active=15212&mapCenter=-8.43883%2C52.42166%2C18&mode=explore
It might include (part of) a field system (LI040-113----), though it is unclear to where it extends. Personally I only see the linear strip along the boundary. I doubt it is (nearly) circular, though there exist examples of those around enclosures.

If you think there is a possible archaeological site, you can mark it with 'maybe' lifecycle prefix and add a 'note' tag to describe what you think it might be and whether it has been reported/confirmed by NMS (which can be done using their monument report form via email).

Regards,

Daniel