If you drink, do not drive. The Irish Laws to Driving Under the Influence have become quite strict and breadth tests are applied even during morning times.
Do not get involved in any way with drugs.
Do check beforehand when hiring a car or other vehicle if there is proper insurance cover and if it covers injury to third parties.
Do travel on a return ticket and bring enough funds with you as bank transfers can take time.
Do take out medical insurance. Medical treatment is not free in Ireland and hospital bills (and medical repatriation) can be extremely expensive.
Do remember that you are a guest in the country and respect Irish Law and traditions.