No evidence has ever been produced to support the mound theory, and in my opinion, limited though it may be, they never existed. The dolmens are a striking feature in the landscape, and were meant to be seen. To go to the considerable effort to build them in the first place (and if you ever get to see the example at Kilmogue in Co. Kilkenny, one of the most remarkable pieces of balancing in western Europe, and one that would defy the most modern structural engineer) and then to cover them with earth, seems ridiculous.