Shannon Airport

I'm a little OCD but I like to show up at 2.5-3 hours early for a flight so any problems can be worked out before the flight leaves. Even though we try to use a check list, I remember one time when we arrived at the airport to learn that my teenage son had forgot his passport, so we had time to make a quick run home for the passport! There have been other times when a flight had been cancelled and we were able to get on a earlier flight so we made all our connections. I am happy to just sit and read once I'm at the departing gate with my carry-on so the extra time is not a issue, so people just don't like waiting and this is not an option for them. The last time we flew back from Dublin we arrived about 2 hours early and just made it through all the security for the flight to Atlanta. If your flying back from CDG in Paris I would definitely arrive 3 hours early as it can get very crazy at times.

Shannon is very small, so I think if you plan to get there by about 9:00 you will be fine. Are you dropping off a rental car? Allow a little extra time for that, since you will do paperwork, then most likely have to be shuttled to the airport from the rental dropoff. Your hotel or B&B might be able to printer your boarding passes for you. If you end up with extra time once you are there, you can always browse the shops or get a snack.

Remember that you will pass through US Customs and Immigration at Shannon Airport, so that adds a bit of time. Aer Lingus recommends 3 hours and it is hard to dispute them. When you arrive at PHL, you'll just pick your bags off the carousel and go, just like a domestic flight.

We took that same flight in June and started checking in at 9:00 am. That gave us plenty of time to get through security and US Customs with a little to spare.

I would allow 3 hours
car hire drop off
Check in depends on how many flights are checking in at same time
Security
US immigration again depends on how many US flights are being processed

Hope you had a good trip