Not sure there is such a thing as a good exchange rate as it is a variable transaction. However, do some searching on Google or whatever you use. Personally, I prefer to exchange before we go. Lots of places in the UK, M&S, holiday shops, Post Office, banks.

Ref car hire, I would presume any credit card with the holder present.

Any airport where you "do" immigration before flying means the flight should be classed as an internal flight in the US and yes, you can just collect your luggage and walk through the airport. Haven't been on the Aer Lingus flight, someone who has will post soon I would have thought.

Have a good trip.