Irnstad was a Dragon Rider that lived before Galbatorix's reign. He was one of only four beings to slay a Shade and live to tell the tale, the other three being Laetrí the elf, Eragon and Arya. Judging solely from the composition and sound of the name, one could suppose that Irnstad was a human male.
The male part is quite likely, while the human/elf question is somewhat fuzzier however: Irnstad bears strong resemblance to real-world nordic names (or at least of Teutonic origin), which seem to be the basis for many human and few elven names encountered in Inheritance. Compare Irnstad/Risthart-Martland-Ulhart-Brom-Tornac/Trianna-Lorana-Marcus-Carn-Tara-Morn/Laetrí-Islanzadí-Arya-Vanir-Lifaen-Narí. The first group (after Irnstad) are decidedly Teutonic-sounding names. One might encounter similarly named individuals in Germany, Norway or Denmark, all countries with a strong Teuton heritage. The second, added for contrast, are human names from Inheritance that hint at a Gaelic/Latin origin, the other significant group of human names in the books. The last group are assorted elven names - all of these seem to be quite Tolkienesque (which in turn are an innovative mix of ancient teuton and anglo-saxon).
However, Irnstad's race and gender are never mentioned in the books.