Hello!
That's great to hear you're planning a trip to the island - and to such a lovely corner of it too!
If you hold a valid US driving licence, you generally won't need an International Driving Permit (IDP) to rent a car in Ireland. However, it's always best to double-check directly with your chosen rental company before travelling, as requirements can vary. You can find more details about renting a car
here.
In terms of driving on the left: visitors often report that it's actually easier than it first sounds. A good tip I like to share is to first practice in a car park to get the hang of it. Having the person not driving help out and regularly remind you to drive on the left can also be useful. Finally, just take it easy and drive slowly. You'll also find that local drivers are reputed to be patient and helpful. If you're used to driving automatic cars, I'd also recommend sticking to that option if you can.
I hope that helps, but let me know if you need any other advice!
Alex