The accent/clothing etc are cultural markers, not racial... also, I can't stand the "lilting voices" description of the Irish. We don't have "lilting" voices, as far as I can tell (what the hell is lilting, anyway? Going up and down?). We tend to have a sort of grumbly voice, rambling on like whatever....
While race, defined as skin colour, seems to have little relevance in 40K (though a frontier world or an isolated world could certainly begin a pogrom of all people with a certain skin colour on the claim it is a mutation, and thou shalt not suffer the mutant to live), race in the broader sense of a genetically related group distinct from other groups with the human species is definitely there - Catachans are all considered thuggish, dangerous, but reliable in a tight spot, Cadians are methodical, Galathamorians are religiously superior, etc... Never mind that that can't be true of all cases, these are the stereotypes the Imperial populace will hold, and they count as racial stereotypes.