Is rail available from Shannon airport that connects to Ennis and Limerick ? How about car rental agencies? Cheaper here than Dublin ?
There is no rail service at Shannon. Your best bet is to rent before leaving the US. I use Irish Car Rentals, but all of the major players are there. It's a small airport out in the country, so it's easy to get used to driving on the other side of the road there. I have rented at both Dublin and Shannon and would prefer Shannon if it suits my trip because it's just so much easier there, both to get in and out, and to get to your rental.
Can't tell you about prices but it was so easy at Shannon, you just walk out of airport and car rental was there, in Dublin we had to take the van to car rental agency, both times used Dan Dooley and found them great to work with.