Perfect casting, period. Forget about being Irish or Itilian, its how your portray your character.

Deniro was perfect.

Jimmy Burke was a homicidal maniac in real life. However, he was also a nice guy to some earning the nickname jimmy "the Gent" Burke. Deniro portrayed this perfectly. Many times throughout the film you say the characters soft side and the homicidal maniac side.

Someone is only "miscast" as you say if they did not do a good job in their role. I dont believe in someone being "miscast". They are either good in their portrayal of a character or they are not.