I just returned from a 29 day trip to ireland. Before leaving I researched this question extensively. The World Mastercard will cover you in most cases. It's worth checking with them to make sure that your bank is in the program. Also, have a letter of coverage. The car rental people don't care how you provide coverage. They are going to make a sizeable hold on your credit card. If you return the car undamaged they release the hold. If there is ANY damage they will charge you for the repair, the towing and the loss of use. Be sure your World Mastercard does not cover just the damage portion of the bill.
If you do have damage to the vehicle they will charge you and you will file a claim with the credit card company's agency in order to be reimbursed.

Personally, I went with Dan Dooley Car Rental. They have a US location and they are one of the few rental companies that have full disclosure of all fees upfront. I accepted their Super CDW coverage as well as glass and tire coverage. My deductible would have been €100. I did incur some scratches on the passenger side door- it was a hedgerow or a tour bus and I chose the hedgerow. So far they haven't billed the €100.