It is an energetic, picaresque story of an American student in Ireland - a charming psychopath, drunkard and rake; written in prose-poetic style reminiscent of James Joyce or Flann O'Brien.
As a novel it is innovative and influential - and being sexually explicit (albeit linguistically hard to unravel) was naturally a big success; probably because it did much to make selfish-short-termism sound attractive and enjoyable!
This was the much-reproduced publicity and jacket photo of Donleavy - presumably taken in the 1970s:
I wonder whether Donleavy came across this cover; and if so what he thought about it. My guess is that he would have seen the appropriateness, and indeed welcomed it. After all, from what I gather of his life and from his books; I don't suppose that JPD would have harboured any serious or sustained desire to 'ascend into Heaven'.