Yes Sir Furry, I may have found for you who Ivan was, of Pentre Ifan!
...and again, it seems the Welsh started just about everything everywhere, because according to this commenter, "Bowen" is a surname of Welsh origin, and a clan of Bowens was in the tiny town where I grew up in SW Washington State, maybe 40 miles from where I live now...okay here is the comment from Eric Bowen, some relative to your Ifan of Pentre Ifan no doubt...
"'Ifan' is pronounced "EE-van." "Pentre Ifan" means "Evan's Village," and is named after Ifan ab Owain (English spelling: Evan Bowen), sire of the Bowen clan that helped put Henry Tudor on the throne of England."
...and here is an interesting YouTube minutes for you too, showing what the burial site may have looked like when it was built around 3500 B.C., which is more than 5,500 years ago now...WOW! Oh and btw, I also learned that it wasn't really such a big deal to stack those tonnage stones up like that, coz humans were not the ones that did it at all, it was surely done by some Welsh faeries; they had red shoes and red hats, and what is more you better not dance around with these faeries on a night of the full moon, because you would never be able to stop dancing and then the faeries would just whisk you right away....okay here is that computerized reconstruction of Pentre Ifan dolmen when it was brand new.