I did some more digging -- and I know this might be a bit of a stretch -- and found some information that just might fit with Bones' theory and what we already know about JZB's Basque heritage.
(By the way, you might want to stop reading if you haven't read Bones' thread yet as this might contain possible spoilers!)
"According to Irish mythology, Donn, or the Dark One, is the Lord of the Dead and father of Diarmuid Ua Duibhne, whom he gave to Aengus Óg to be nurtured. Donn is regarded as the father of the Irish race; a position similar to that of Dis Pater and the Gauls, as noted by Julius Caesar. Originally, Donn was the chief of the Sons of Mil, a mythological race who invaded Ireland, ousting the Tuatha Dé Danann."
A little more searching on the Sons of Mil brought up the Milesians:
"Milesians are a people figuring in Irish mythology. The descendants of Míl Espáine, they were the final inhabitants of Ireland, and were believed to represent the Goidelic (or Gaelic) Celts."
According to another site, Míl Espáine translates to "Soldier of Hispania." Also, a recent article in the UK paper The Independent stated that Celts descended from Spanish fishermen. As we well know by now, the Basques hail from Spain. Could this be the link that connects JZB to the Basques?