Get the insurance. Most credit cards have a list of countries they will not cover in and Ireland shows up on those frequently, so check with yours.
Credit card insurance, if a claim is filed, will likely run through your own auto policy first and pick up any difference, so that claim will be on your auto policy record. Also, car rental companies both in the US and abroad tend not to just charge you for the actual damage, but also for lost income while the damaged car is out of service. That can be considerable.
If you opt not to take insurance most car rental companies overseas will put a high authorization hold on your credit card that can be the as much as the cost of the car, so that can impact any other spending you want to do with that card.