A dolmen, also known as a portal tomb usually consists of two or more upright stones called Portal Stones supporting a large flat horizontal Capstone.This construction makes a tripod-shaped structure. In Ireland Dolmens date from as far back as 4000 BC. Cremated remains of the dead were placed inside the Dolmens and then covered over with a mound of small stones. This mound is called a Cairn. In most instances the Cairn has weathered away, leaving only the tripod stone structure of the burial mound intact.
In this video you can get a feel of the size and shape of a Dolmen.