Dolmens or portal tombs?

We had a nice excursion yesterday to Slieve Gullion, the mountain in South Armagh, including the Ballyward Dolmen on its western flank:
The Ballyward dolmen, on the slopes of Slieve Gullion.

My icon for this post is a picture of a favourite County Down attraction, the Legananny Dolmen, taken nine years ago (it’s a lovely place and I went there last summer too):

A sign at Ballyward, just in front of an area of tumbled stones beside the dolmen, solemnly puts forth the view that dolmens were originally part of a larger structure, and were covered wth earth which has worn away or small stones which have been removed.

I have to say I find this profoundly unconvincing. My gut feeling is that these monuments were originally constructed to look much as they do now – striking salutes to the human spirit against the landscape, showing stability over unimaginable lengths of time.

Occam’s Razor surely shaves the notion that they were originally covered by turf or stones pretty close. There are plenty of earth-covered monuments which retain their covering millennia later. And if people over the centuries were removing small stones, why not the big ones as well?

Is there really any evidence that dolmens like Ballyward and Legananny were meant to look different to the way they look now? And if not, why do people believe that they were?