Dolmen at the Craggaunowen Project in County Clare, Ireland. A dolmen, also known as a portal tomb, is a single chamber megalithic tomb. This dolmen consists of number of upright stones supporting a large flat capstone. This type of tomb dates from the early Neolithic period (4000-3000BC). They were usually covered by earth or smaller stones to form a barrow. They generally contained cremated remains and burial goods including pottery and flint tools.