Norse and Celtic (again)

I commented about two years ago about parallels between Norse and Celtic lore, picking up some thematic contrasts and comparisons. Themes of folklore have often been shown to be international, with the same motifs cropping up in different contexts in different cultures. In this sense the fundamentals of human stories do not vary enormously across cultures. But is there likely to be any  cultural similarities between speakers of distinct languages? Many have thought there should be on the basis of them inhabiting similar geographical areas and also a perceived similarity between the art of the two cultural groups. The argument against is the fact that the languages  are distinct. Cultural links between speakers of Danish, Norwegian, Swedish and Icelandic are clear on historical and geographical criteria but also on the fact that the languages are very close to each other and to an extent mutually intelligible. Similarities with German and English, though more distant, are reflected in common mythological backgrounds. With the change of language to Celtic, the mythological background also seems to be distinct. The origin of Celtic speakers may be unclear with identified sources in central Europe and/or the Iberian peninsula. But it is clear that in historical times the two groups overlapped in parts of north-west Europe.

To make this even more complex, there is a debate within Celtic Studies as to the extent of interaction between Welsh and Irish speakers in the historical period and so on the question of a common inheritance for speakers of languages that were no more mutually intelligible than Welsh and Old English. But interaction there must have been between speakers of different languages both on the British mainland and farther afield. Consider that the character who gives his name to the Icelandic Njál's Saga has a name that is Irish in origin. Vikings moved between Ireland and different parts of Britain interacting with both Celts and Saxons. Is there any evidence of anything they have in common apart from aspects of their material culture which might be expected to exhibit similarities given that they inhabited overlapping geographical areas in the same historical period?

In the Völsunga Saga it is said of Sigurd that "he is known in all the tongues north of the Greek Ocean and so it must remain as long as the world endures". I seem to remember something similar being said as the reason for the many different names of Ódinn, so that many different people would be able to know him. "All the tongues north of the Greek Ocean" is certainly pretty inclusive. But differences in language can also be exclusive. Is there a way across this divide?

A recent book* discussing some work done by Tolkien on similarities in e.g. i-mutation in English and i-affection in Welsh led him to the conclusion that " the different languages of North-Western Europe functioned, despite their differences, as in some ways a single philological province, subject to the same influences." Could such a flicker of commonality be of enough significance  to throw light  on deeper cultural currents that might unite these disparate language groups. Or did Matthew Arnold get it right when he pronounced on the contrasting temperaments of Celt and Saxon?

*Tolkien and Wales by Carl Phelpstead