This page concerns Chambered Cairns, also known as entrance graves. While they might have been used temporarily as morgues, it is unlikely they were used as graves. It's more likely that they were used as repositories and initiatory chambers.
They date probably from the early-to-mid 2000s BCE, the late neolithic. Often they are called Scillonian chambered cairns since there are many of them on the Isles of Scilly. But current research reckons they originated in West Penwith, before the Scillies were permanently settled, even though they are less common in Penwith.
For more on Chambered Cairns, try here.