Was there any reason as to why he disliked Frank McCourt so much?

Harris felt that McCourt's portrayal of Limerick was unflattering and felt that the reality of McCourt's childhood was a lot better than what was portrayed in the book. McCourt hit back at Harris claiming Harris came from a more prosperous part of town and couldn't possibly know how he lived. Harris refuted this by saying "McCourt was very well versed in telling the press how well I lived. If he is so well informed about my life why is it unnecessary for me to be informed about his life?"

Harris also felt that McCourt had been cruel and disrespectful to his his mother Angela. Harris reported that he has spoken to Angela and she said Frank and his brother Malachy disliked her and wouldn't visit. She was cremated contrary to her wishes. Harris noted that she was a devout Catholic and wished to be buried. Harris also claimed that McCourt has lost Angela's ashes and that they were "buried away in some far distant remote lost property corner of Kennedy Airport in New York". McCourt denied this until after Harris died and then admitted that he had left his mother's ashes at a bar in New York

http://www.limerick.com/angelasashes/controversy7.html