Shirley is Shaun from A Close Shave.
Look here, Shaun is an eating machine of, err...MASS EATING! in A Close Shave, but in this series, "Shaun" is not that hungry, while "Shirley" EATS AS MUCH AS SHAUN in A Close Shave
Therefore my theory is "Shirley is Shaun". "Shaun" is just Shirley's son(, and Shirley doesn't have to be a girl - It says this on the Aardman website). Shaun changed his name and gender (on his identity ONLY!
) so Wallace and Gromit wouldn't find him.