There is an item on the CADW website which shows how Pentre Ifan may have looked when first built using CGI.
Worth a look if you have a few spare minutes.
https://www.youtube.com/watch?v=Ulip6oUxqDs&feature=youtu.be
In the Aubrey Manning series 'Talking Landscapes' shown originally a few years back there was one about Pembrokeshire which had a reconstruction of Pentre Ifan covered by an earth mound so who's to say this latest suggestion is right?