There are 3 of us, and we will be in Portrush, Northern Ireland without a car. From Northern Ireland we need to get to Kilkenny, Ireland. We will need a car in Kilkenny and for the remainder of our trip through Ireland. My question is where should we rent a car in Ireland? Dublin, outside of Dublin or take a train all the way from Portrush to Kilkenny and rent a car there?
Depends where you will be returning the car, but you could take a bus from Belfast to Dublin Airport and rent a car there.
We will be returning our rental car in Dublin. Taking a bus to the airport sounds like a good plan. We will already be on a train from Northern Ireland. Is there a train that goes to the Dublin airport?
No - you need a bus. If you disclose where you train ends we can help better. If not you can find information on https://www.dublinairport.com/to-from-the-airport/by-bus