Taken just as you've written it, without examining the context, you'd need to center on "scrupulous meanness." Joyce says that he wrote Dubliners with scrupulous meanness.
Scrupulous means to have in mind what you believe to be right. It also means exactness and precision. So Joyce says that he wrote what he thought was right, precisely and exactly. He might also mean that he arrived at what he thought was right by exact and precise study.
Understanding what Joyce means becomes more difficult, though, when one adds meanness to the thought. He could mean one of two things, or both. First, he could mean that what he thought was right led him to meanness. Second, he could mean that he was not mean carelessly, that his being mean was the result of what he thought was right.
In short, the stories are the products of what Joyce thinks is moral or right. One might say that they demonstrate Joyce's "righteous indignation," without the usual religious, simplistic, and moralistic connotations those words might usually suggest.
In Dubliners, the shortcomings of Dublin residents, and the Irish as a whole and humans in general, are harshly exposed. Humans can be ignorant, superstitious, manipulative, psychologically impotent and paralyzed, etc. These weaknesses are exposed in the stories. One could conclude that the stories are written with "meanness."
Yet, the stories are not didactic--preachy or sermon-like. Joyce's narrators do not intrude (and neither does he as author) to condemn Dublin residents. Joyce does not appear to directly condemn anyone. Instead, weaknesses are revealed by narrative.
In short, Joyce exposes weaknesses, but, he says, he does it based on what he believes to be right, and he does it with exactness and precision. He is mean, but his meanness is based on what he carefully considers to be right.