The whole thing turns out to be a very awkward misunderstanding.
According to an Aston Martin spokesperson, no one within the company was fired over the mishap, confirming a report by Jalopnik to the same effect. The name "Dirty Shirley" was scrawled on the prototype's rear end by a German supplier; however, Aston says the name simply turned out to be a poor choice in retrospect, as when the supplier wrote it, said supplier was unaware of the lewd Urban Dictionary definition. No one at the supplier was fired, either. And those tally marks on the side of the car? Simply the number of times the prototype had lapped the Nürburgring.
So it turns out the whole thing was a big misunderstanding. Honestly, given the sheer number of entries on Urban Dictionary, it's a wonder an automaker can call a car anything without the name turning out to be a synonym for some less-than-polite act somewhere in the world. (Seriously—not the first time this sort of thing has happened. Maybe that's why so many car makers have gone over to alphanumeric names...) Anyway, glad nobody lost their job, and glad we can put this story to rest. In the meantime, enjoy a couple images of the One-77 all cleaned up.