I was doing my Ancestry and did some research and the only thing we know of his life in Ireland is that he lived in southwest Ireland due to people who still have his last name sharing most of their DNA with people still there. His last name was Fitzgerald when he moved to the US but he changed it when he got here. I havent found any records that state his previous name
You might want to ask this over on r/Genealogy where they are more into ancestry records.
I’m afraid your question, as you pose it has no answer. No one other than your great grandfather will probably ever know his true motives for burning records and changing his name. Maybe he was a criminal, maybe he just wanted a fresh start, maybe he was worried about the way Irish were often treated and wished to leave his linage behind. I would start with the r/Genealogy subreddit and someone might be able to help you locate record offices in Ireland for things he might not have been able to destroy.