Basque and Irish populations have an extraordinarily high proportion of R1b males, with few other haplogroups, and are thus thought to represent the earliest peoples to settle Western Europe.  They are simply the most unmixed, because of their relative isolation.

A major reason so many males from the British Isles have matches in Spain, Portugal and the Basque country, is that the Iberian peninsula was a crucial zone of refuge during the height of the last Ice Age, and it was from there that the Atlantic coast of Europe was repopulated once the glaciers receded and allowed the North to be inhabited, around
10,000 years ago. Combined with what is known from archaeology about prehistory, a lot can be inferred about why there are such strong similarities between many people from Spain, Portugal, the Basque country (and regions like Galicia, Andalusia, etc., etc.), and Atlantic areas that lie north from there.

Bonnie Schrack
Excerpts from Genealogy-Dna-D@rootsweb.com 26th June 2003