Sink Hole, Impact Crater or Turlough?

Image courtesy Google Maps and Maxar Technologies

I've often wondered about this circular pond area at the back of what's called "Brownstown House" on the first edition c. 1837 OSI map or 19 Mile House Inn on earlier maps, once the meeting place of 1798 rebels and a home of the McDonnell family in past decades. The Geological Survey of Ireland interactive map however doesn't show any magnetic anomalies which I think would be the case if it was a mini crater site, meteorites often being made of iron. But not all of them are. Maybe the circular shape is man-made or a natural phenomenon due to the edges falling in uniformly all around the perimeter because of erosion and sheep wandering around it over time.

Does anyone know anything about it?
Maybe it's a turlough?

 

There is a red "hotspot" of magnetic intensity further east, but I think that may be due to the covered KTK landfill (Planning permission has now been lodged for a solar farm there). When I zoom into the map, the shading disappears, so I can't identify whether it aligns with the landfill. I've queried this bug with the GSI.

 

Edit: This EPA report from 2011 mentions "..reclamation of metals and metal compounds" at the KTK landfill.

 

https://epawebapp.epa.ie/.../lic_eDMS/090151b2804386da.pdf

 

A close-up of the pond. Image courtesy Google Maps and Maxar Technologies

  

6" c. 1837 map, courtesy OSI (Tailte Éireann)

The red area shows a region of higher magnetic intensity. Map courtesy GSI (Tailte Éireann)