Help to identify structure.

While walking in Dysart woods near Portlaoise i came across an unusual structure.
It is built into a slope. From above there is a circular hole roughly twelve feet wide. Underneath there is tunnel like stonework, which although filled now with vegetation looks like it led into the cavity from above.
A few days later i found another very similar structure about 3km away. This one appears to have been built later with far better stonework. The hole on this one is smaller at about 5 feet wide.
My guess is that these are charcoal pits?
If anyone has information about these i would appreciate it.






This picture is from above. Its hard top make out the hole because it is overgrown.



These pictures are of the newer looking structure.