What kind of accent is Shaun’s girlfriend’s twat of a flatmate who loves her have?
Dylan Moran? He is Irish.
Makes me want to put the film on now just thinking about it. 
I just want to say thank you for starting this thread. I just saw Shaun a week ago (amazing movie, btw) and this exact question was bugging me.
If you’ve seen his show “Black Books”, he’s very Irish.
Black Books is superb. In Shaun of the Dead, Dylan Moran just plays a, well, twat, and does it well. He is much funnier in Black Books.