Found it. It's called a Fulacht Fiadh (cooking place in the wild) and was used by hunters while camping. According this site
http://www.inhp.com/ the same campsites were used on a regular bases and they were usually near water marsh, river, lake etc.
The animal was skined and then the meat wrapped in straw. The water was brought to the boil by using heated rocks and the meat put in. Accord the tests at the heritage centre it takes 20mins per pound + 20 more pretty much like a modern oven. They don't say what meat they used do.
I have been here in Winter and was pretty dead but I have been told it's lot more active during the Summer.
James