Of course a design feature can be based on multiple sources of inspiration at once. Virtually all of Star Wars is a mish-mash of various pre-existing stories. But that film appeals to us because the old stories are combined in a way that makes them fresh and interesting.

Lucas may indeed have been drawing on Celtic legend; in fact, I'm sure he was. But he definitely drew directly on Tolkien as well. I mean, come on, he plagiarized the opening of The Hobbit verbatim in his third draft script for SW 1977.

Most of the names in Willow, not just Madmartigan's, are actually reworkings of other character names from pre-existing sources. Sorsha is named for Shasta from CS Lewis's The Horse and His Boy, "Willow Ufgood" has the same meter as "Frodo Baggins," and the last name of Airk Thaughbaer is a garbling of "Théoden."

In fact, in the third-revision script of the film, some wild tribesmen show up who are called Picts. In the novelization and comic adaptation they also appear, but are renamed Pohas. This is an extremely clear example of Lucas taking a pre-existing literary name--the Celtic tribe was famously fictionalized by Robert E. Howard--and slightly altering it to obscure its origins in another work.