where do people return car rental at Shannon International Airport

You return it to an offsite location right next to the airport and take a shuttle to the airport itself. You can actually almost see the lot from the airport. If you're not renting in Shannon to start with, your rental place should give you directions to the Shannon dropoff. It's easy.

I'll piggyback on Norma's question and ask about the size of the Shannon airport. Is it a busy place? For a 9:00 A.M. flight back to the USA, does arriving at 6:30 A.M. give plenty of time to return the car, shuttle to the airport, check in, get though security, and get to the gate by 9:00 (or by 8:30...I know some places close the gate 30 minutes before departure)?

Be sure you check what time they open. We showed up at Glasgow for a Hertz return at 6:30 and then waited until 7:00 when an employee showed up to open the return lot.

Shannon Airport is small and not overly-busy. Great airport to fly in and out of. Car rental return is very close. I've arrived at the car rental return 2 hours before my departure and had more than enough time to sit around and get bored at the airport.

Shannon Airport is about 1/2 the size of a Wal-Mart store.

If I remember correctly, it took us about half hour, 45 minutes at most, to return our car, get to the airport, spend our remaining Euros on food and reading materials, get through security and get to our gate.