That is not the silliest idea I've heard. Arthur's foster father was Sir Ector in the legend, and the medieval Welsh lords when youths, were often raised by their uncles. Sir Ector equates to another legendary character, Cynyr Cainfarfog, and Irish lord of Caer Goch in Dyfed. Dyfed was almost certainly under Irish control in the 5th century. If the myth is at least partially true, Arthur's father may have be Irish, specifically from the Deisi tribe.
https://en.wikipedia.org/wiki/Cynyr_Ceinfarfog
https://www.earlybritishkingdoms.com/bios/ector.html
https://en.wikipedia.org/wiki/Déisi
Arthur's mother was called Eigyr. In Welsh, that means chaste or pure, and suddenly we have a Christian link to Mary, mum of a certain famous figure. That in turn casts doubt on the whole claimed parentage of Arthur.
http://www.earlybritishkingdoms.com/bios/ygerna.html