Here are two examples of dolmens, both in Co. Down. They would have contained burials or cremated remains, presumably of prominent persons. I have posted three shots of Leganannay, to give an idea of the structure. The second dolmen on the slopes of Slieve Donard is a pretty rough photo, but enough to give an idea of its appearance. It is presumed that the spaces were once made up of packed earth and stones, leaving a tidy mound shape. Erosion over the millennia, together with borrowing of smaller stones by local farmers over 4000 years or so has resulted in the stone skeleton we see today. The massive capstone was a de rigeur feature of a dolmen. The biggest one in Ireland is said to weigh 200 tonnes. The ones portrayed are much lighter, but still a challenge. Builders had to make do with available stones, presumably deposited by retreating glaciers. It looks as if there was a prohibition on the use of tools to improve the shape.