@Robert ********
I lived in Belfast for years. Gard never stopped me for papers in a UK car, I was stopped by the police more often up north. Even when the border was armed, and during FMV, I could cross the border freely. If you flew into Belfast and drove south in a rental car, no one is going to stop you. Get a ferry from England or scotland to Larne or Belfast, you dont need a passport. Some, not all, Airlines and Ferries going directly into the Republic might set a passport requirement, but that's not an Irish government requirement. The Foreign Office advice is you should mot must have a passport. If you don't have a passport, you can obtain a Confirmation of British Nationality Certificate from HMG Incidentally, Thailand is included in the Short Stay Waiver Scheme, so some with a UK visitor can visit Ireland for a short time with needing an Irish visa.