It's so exciting you're planning on visiting us soon! I'd love to point you in the right direction.
You can find lots of helpful information about renting a car in the I'm thinking of renting a car in Ireland - what do I need to know?
thread. Have you planned out your itinerary
yet for those 3 days?
There are some car rental companies that do not require a credit card, but in that case there are additional requirements for the insurance. Take a look at the Car Hire without a Credit Card
thread from Carlien for these companies and more helpful resources and information.
If you reach out tot the rental company of your choice they will gladly tell you more about the possibilities, travelling between the Republic or Ireland and Northern Ireland, the costs, drop off locations and registering a second driver.
Keep me posted on your plans, and I hope some of our other members can provide you with more tips and suggestions! :)