I do think that the Holly King idea has been around over in Wales for a long long long, time.
Sorry to disappoint you, but it really hasn’t. Robert Graves drew on some ideas from Frazer’s The Golden Bough, but basically the whole idea of a Holly King and an Oak King is one of Graves’s inventions – it’s not really rooted in traditional folklore. I’m not saying it’s not valid or interesting, but it needs to be borne in mind that it’s pretty much a modern notion.
Graves was not all wrong, he did a lot for magic and druidry. So, I am not a Graves hater.
… but mostly he was, and the problem is that many of Graves and Frazer’s ideas have become so rooted that people accept them without questioning. It’s not about “hating” someone, critique of scholarship and work is not a personal attack on the author.
Hunting the wren is a bit strange because at least in Cornwall, the robin and the wren were usually considered taboo in the sense of not to be harmed, it’s paradoxical that the wren hunt existed, albeit on one day of the year. The tradition, however, may not be all that “Celtic”, similar traditions are also found in different parts of France and Spain. Who knows? Nevertheless, as much as we find it abhorrent today, practically all ancient societies performed some form of animal sacrifice, including Celtic societies.