One of the most thoughtful recent scholars on this subject was the late Bo Almqvist (1931-2013) - a Swedish folklorist who was the director of the Irish Department of Folklore at University College, Dublin. His "Viking Ale: Studies on Folklore Contacts between the Northern and the Western Worlds", edited by Éilís Ní Dhuibhne and Séamas Ó Catháin (Aberystwyth: Boethius, 1991) is an excellent collection of articles, many of which address what you are asking. Here is a quote that helps frame this discussion:

[The Norwegian folklorist Reidar Christiansen] has pointed to a series of close parallels between certain Scandinavian and Scottish-Gaelic and Irish legends, and has introduced the term North Sea legends. It is not easy to account satisfactorily for these similarities. It may be that some of the motifs and legends are part of a common stock, but there can be little doubt either that the Norse and Gaelic speaking communities influenced one another, and that certain types and sub-types spread in either direction.

What is true of folklore is equally true of art and various other cultural motifs. We cannot speak of something like Newgrange without acknowledging that it is pre-Celtic, but the point is well taken - what seems to be elements of common cultural inheritance also seem to pre-date both the Celts and the Scandinavians - as we understand them. The question remains - why do these similarities exist? Diffusion through various periods of contact seems like an obvious answer, but aspects of shared cultural elements seem too deep to be accounted for strictly through diffusion.

I address this to a certain extent in my forthcoming, "The Folklore of Cornwall: The Oral Tradition of a Celtic Nation". The following is an excerpt - before the copyeditor makes me seem far smarter than I am (so apologies for any clunky language!):

Almqvist addresses a question in British studies, looking to those who conclude that Anglo-Saxons invaders gave as much as they took, creating a hybridised society. Almqvist applied this to assist in his life-long study of the interaction of Celtic and Scandinavian folklore. In his essay, ‘Scandinavian and Celtic Folklore Contacts in the Earldom of Orkney’, he attempts to determine which traditions were borrowed and which may represent common inheritance. It is no mean task, but Almqvist writes with authority, having spent decades pondering the question, reading sources in multiple languages to consider possible origins of parallel texts.

Cornwall presented a similar situation since it encountered not only Anglo-Saxons but also Scandinavians. Cornwall cannot be regarded as the ‘Venice of the North’, as Almqvist characterises the medieval earldom of Orkney; nevertheless, the Cornish nation was also something of a crossroad. While it resisted medieval Anglo-Saxon advances, Cornwall was not as secluded as the hinterland of Wales or north-western, mountainous Caledonia. One would expect that the Cornish had their own oral tradition before the many forces of history had their way. Regardless of how factors affecting lives and culture played out, the folklore of Cornwall certainly exhibits the same common inheritance that Almqvist suggests ‘is part of the explanation of the unity that exists today among the peoples of the British Isles.’

What can be said about similarities throughout Britain and Ireland, can be extended effortlessly, also, to Scandinavia and Iceland. In otherwords, yours is an excellent question, and many have pondered the answer(s) with various degrees of success.

To clarify, I don't just mean 'Celts' and 'Norse/Nords', I mean any and all peoples from Ireland, Britain and Scandinavia.

Prehistoric peoples, Gaelic peoples, Anglo-Saxons, Vikings, etc.