It’s suggested that the rather Scandinavian name of these burial chambers is after the supposed founder of Swansea, Sweyne Forkbeard, (Svend Tveskæg) King of Denmark and sometime king of England 1013-14. Swansea is first mentioned as “Sweynesse” in a 12th century charter. The Welsh name for the city is quite different and sensibly refers to the mouth of the river (Abertawe).
“We all know” the chambers are really prehistoric and not Viking at all – but the mounds might well have been recognised and even reused for a burial in later years: many others across the country were. The story is that Sweyne himself is buried here, but you’d like to think he made it back to Denmark really.
There’s an Earl Sweyne going to Wales here in the Anglo Saxon Chronicle in 1046. So maybe Sweynes were just two a penny at the time.
worldwideschool.org/library/books/hst/english/TheAnglo-SaxonChronicle/chap11.html