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This is very interesting.
Although I won't contribute directly to the topic with this, Brittany and Central Portugal share the same type of megalithic architecture, as has been noted in 1977 (here, section 5); which, at least in Brittany, is considered to be of such antiquity, previous to any known neolithic migration into the area, as to suggest the idea of "proto-megalithism".
"In three regions, the Tagus valley, Morbihan and the Irish Sea, there are remains of shell middens. The first two of these groups are also associated with Mesolithic cemeteries" which, according to one hipothesis (see link above) would be the starting points of the people who, after migrating inland, built the megaliths.
The much later Iberian (or at least, Lusitanian) genetic mix with the incoming Celts originated the Celtiberian (Iberian Celts).
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