Landing at LGW at 12:40pm is it possible to make a 7:35pm flight from LHR to Shannon that day

Seven hours should be plenty of time to connect. Doesn't sound fun to me to have to move around like that and then hurry-up-and-wait (almost enough time to stop in London for a few hours - almost), but other than the inconvenience and long layover, I don't see any problems.

You could also just camp out at LGW for eight hours and take the RyanAir flight at 8:25p from LGW to SNN. No idea whether there is luggage storage out at LGW anymore - but if I could leave my bags at LGW for a while, I'd probably take the train into London for an hour or two. If you have to go to LHR instead you'd need to cart your bags with you.

London Gatwick to London Heathrow is about 38 miles. The National Express Coach can be a good option to get between airports and takes 1 hr 5 mins. www.nationalexpress.com (depending on M25 traffic). If you want to take the train then you will have to connect through central London. I like to take the bus its more relaxing after a long flight.

You don't tell us what time you land at LGW. I am assuming you land sometime in the morning. If your flight lands on time - I would add two hours on for passport control etc and to get to the buses/trains. Then you have to be at Heathrow about three hours ahead of your flight. Keep in mind these airports are very busy and lines can be long. If you are re-checking luggage and switching airlines you will need this amount of time at LHR - with hand luggage not so much. So you need to be getting to Heathrow about 4:30 pm. If you don't have enough time then stay the night at LHR (Premier Inn at T4) is a good budget choice and then head out the next day more relaxed. Traveling all day to get onto another flight that evening is going to be tiring. It also depends on how you cope with lack of sleep.
Margaret

Andrew, it doesn't sound like much fun to me either, and I agree "almost" but not quite enough time.... Thank you for the Ryan Air flight, it didn't show up in my flight search.

Margaret, We're landing at 12:40pm.

Thank you both for the advice, trying to decide if I want to "get it all over at once (despite the long layover)" and go to Shannon/Ennis that first night and get settled in for the 2 pretour nights, or wait and go in the morning.

LyndaC.

This is a hard decision - its depends on how active your are and how much you can tolerate sleep deprivation. Its already a long flight from SFO to LGW. With that in mind, I might be inclined to take the connecting bus and book an airport hotel for the flight out of LHR into Shannon for the 21st. Aer Lingus has four or five flights each day. On two recent trips to Europe, I have stopped at Heathrow for the night before going onto my next destination. Its makes me a happy traveler to have good rest. RyanAir are OK, but I avoid them if I have a cost effective alternative.
Margaret

Once you get to Shannon, you still have to get a bus to Ennis. You will need time to get through immigration at Shannon although they are usually fairly quick. (U.S. passport holders have to go through immigration in Ireland when arriving from the UK.)

According to your plan, you arrive at Shannon at 9 PM local time. The next bus to Ennis, according to the current time table and it might change, is at 9:24 PM. The airport is small and you might make it. It gets you into Ennis at 10:10 pm and from there it is about a 10 minute walk to the hotel. (Assuming its the same hotel they normally use.) If you miss that bus the next one is an hour later.

Personally, I would break up the day...fly to LGW and then get the bus to Heathrow. Check into a hotel, get a good night sleep and fly the following day to Shannon.

I've booked the Aer Lingus flight to Shannon on the 21st. Thanks for helping me see the more comfortable option for starting our vacation off smoothly.