The palace was at Garth Celyn (Clwyd) and the ancestor in question was Llewellyn (Fawr) ab Iorweth, whose daughter Gwladys Dhu married Ralph de Mortimer of Wigmore. So, despite the recently highlighted doubts about the other contender at Bosworth, Richard at least was of royal Welsh descent.
Here is Paul Martin Remfry’s article in full:
https://www.academia.edu/9841761/Llywelyns_Palace_at_Aber_and_other_castles_and_llys
The New Murrey and Blue blog 
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