When he returned to America later in his life, I think it gave him some perspective on life in Ireland, both in terms of the economic hardships he faced, but also on his own family. Being separated from where he grew up by an ocean allowed him to assess his father and mother's strengths and weaknesses objectively, and the stories he tells in Angela's Ashes are both funny and tragic because he has this objectivity.
McCourt was born in Brooklyn, New York, in America in 1930. When he was age four, his family was moved back to Ireland by his mother's family living in Limerick, Ireland, in the Catholic south. He begins his memoir with a pathetic description of life in Limerick, then briefly describes his father ad mother. All he says about his first four years in America--the only time that he was in America that might have had an effect on his later time in Ireland--is that his parents should have stayed in New York: "My mother and father should have stayed in New York where they met and were married ...." The conclusion, then, is that McCourt's time in Ireland was not affected by his earlier time in America, except to form a foundation for lost hope.
McCourt's childhood has a lasting effect on him. In his book Teacher Man, he describes how he told his students with long stories about his "miserable Irish childhood" to make them feel sorry for him. Eventually, he wrote them into this book because people felt that he had stories worth telling.