Verified answer

The breed is never identified in the book beyond being a large black dog. In the movie Order of the Phoenix it was an Irish Wolfhound. In the movie Prisoner of Azkaban, it looked like a wolfhound mix.

Fang, Hagrid's dog, is a boarhound. Great Danes are usually considered boarhounds. But in the movies, Fang is played by a Neapolitan Mastiff.

Irish Wolfhound

Doesn't he become a boarhound? I don't have the books to hand but I seem to recall the breed being mentioned.