This is the Drombeg Stone Circle, also known as the Druid's Altar. It is a "Recumbent Stone Circle" which means that it has one large stone laid on its side. While Drombeg is one of the most visited megalithic sites in Ireland, there are about 1,000 stone circles and 80 stone henges in Britain and Ireland.
The number of stones can vary between four and 60 purposely erected standing stones. The recumbent stone circle is found almost exclusively in North East Scotland. The recumbent stones themselves are almost always in the SW quadrant of the circle, and are aligned on the major moonrise. (Stone circles vary from henges such as Stonehenge, which have a ditch and/or bank of earth enclosing them.)