A girl, usually a hick and or some sort of trash who can be seen: Riding in muddy trucks while wearing a rebel cap, chewing tabacco, scratching their vagina/rubbing her vagina in the dirt after urinating in the bushes, flaunting a dirty thong,or posses any other feature one may feel is "dirty". The McGirk is just a cool ending.
"Hey Ty that girl looks like dirt"
Girl proceeds to turn around and her shirt says "I love dirt"
"holy crap Ty she is Dirt McGirk"