london to ireland and back then to US

As long as you don't leave the secure area of the airport you should only have to pass through passport control upon arrival at Stansted. At Heathrow you may have to pass through security again approaching the gate for your U.S. bound flight so factor time in for that. You will have to pass through customs upon arrival in the U.S. (as well as passport control of course). I'd pad in 4 hours prior to your departure at 1:35 p.m. Might be excessive but how often does a flight leave and/or arrive on time. Then there's the whole changing terminals deal, screw ups along the way (at Heathrow this is almost a given). I'm guessing you already have your airline tickets. If not you can clear customs for the U.S. flying out of Dublin for the U.S. and avoid Heathrow entirely.

Thank you so much for your response. So, to be clear, if we need to check baggage on our flight from Kayseri, Turkey to London Stansted will we have to leave the secure area at the airport to claim our bags? Would we then be required to go through customs before boarding our next flight to Dublin? If so, how much time should we allow between flights at Stansted? And yes, we have already purchased most of our plane tickets...I'm still working on a few short legs of our trip. I heard that we could have by-passed customs in the US coming straight back from Dublin, but unfortunately we bought our airline tickets in pieces. We are adding the Dublin leg of our trip on at the end now and have to change planes at Heathrow. Next time I will think it all through ahead of time. Thanks.

You will need to claim your bags, but you won't go through customs, you will go through passport control (immigration) and security to get on to your next flight. To your comment about bypassing customs if arriving from Dublin to the US, you don't bypass it, you just do it in Dublin instead of in your arrival city. It still takes the same amount of time, roughly. If you are buying your flights one at a time, I can't imagine what you must be paying!

Actually we saved money flying a multi-city flight from MSP to IST and then from LHR back to MSP. I was surprised too. We were able to add one ways via Pegasus Airlines from IST to Izmir and then Kaydseri to LON Stansted that ended up being only a little more than RT from just MSP to IST. Adding in our flights from LON Stansted to Dublin and back from Dub to LHR via Aer Lingus and Ryan Air was only about $350 total for the 2 of us which includes checked baggage fees. In all total we spent under $1800 each, I think.