Thanks for taking the time to post your question about car rental
You are correct when you say that your USA state driving license is sufficient, you don't need an international permit to rent a car and drive on the island of Ireland!
With regards to the insurance, there is no hard and fast rule about the extra cover, we would suggest that you contact individual rental companies
and ask them specifically as their terms and conditions vary. Some have the option for extra cover payable in advance and therefore cleared before you arrive. A credit card is required at the time of renting along with the valid licenses of all drivers.
The Car Rental Council of Ireland
also has a website with some information and contact details.
Hopefully, you can find all the correct details you need, when are you planning on visiting the island of Ireland?