how can a tartan of a specific county be copyrighted?
Whoever designed the tartan can copyright it, regardless of whether it's named after Mayo or Mars.

for someone wanting to honor their Irish roots, how abut a Stillwater solid Green or Saffron. I believe many Irish regiments wore / wear these.
Only Irish pipers wear kilts; saffron kilts. I'd also suggest a solid green or saffron one from Stillwater (I've got a saffron one, and am very happy with it), or perhaps the All Ireland Green from Sportkilts (a friend has one of those; I like the colors better than the Irish National).