well, the obvious answer is that in popular culture these two seemingly similar cultures were conflated into one "Euro-Barbarian" simply for coolness sake, and as such appear in the media, from movies and books to games and art.

A more charitable answer could be that we do have examples of late Celtic (Gaelic) and Norse cultures mingling, especially in the case of the Viking conquest of Ireland (Dublin would be the prime place to look at), as well as Norse settlements in Scotland. If one considers the Gaelic Irish and Scots as descendants of the Celts and the continuity of their cultural tradition (a view that is largely correct, archaeologically), then the Norse-Irish and Norse-Scots resulting from cultural mingling would be both "Celtic" and "Norse" . This is, I think, well supported by both historical evidence and evidence from contemporary art.

Lets not forget as well that the mingling went the other direction as well: The Norse took numerous Gaelic slaves and distributed them all over their newfound settlements, in a way that the "Norse" of the Faroe Islands, Iceland, and other insular places have Gaelic genetics, and cultural elements. This was so common that the name the Norse had for Gaelic people (Vestmen - people of the West) is still a generic surname in these places.

In effect, the shores and islands encircling the British Isles and between the Isles and Scandinavia were to a large part a domain of Norse-Gaels: "nordicised" Gaels as well as people from mixed marriages.
One does not have to look much further for evidence than compare the late Viking artwork, and how much it resembles Gaelic and Pictish artwork. For example, the Urnes Style of Viking art seems to have been copied from Gaelic vine-like knotwork.

Downham, Clare "Hiberno-Norwegians and Anglo-Danes

Fuglesang, S.H. Swedish runestones of the eleventh century: ornament and dating,
Anker, P. The Art of Scandinavia