I am purchasing a ticket on United from San Francisco to Dublin with a layover at Heathrow. The flight is purchased through United but the flights themselves are United from SFO to Heathrow, 1 hr 35 min Heathrow layover, board Aer Lingus for Heathrow to Dublin. Is 1 hr 35 min enough layover time to switch from a United flight from US to Dublin? Same problem on return only 1 hr 10 min layover in London. Because the US - Heathrow -US legs are international, 1: is the London to Dublin considered domestic or also international and therefore 2: do we switch terminals and do we go through customs in Heathrow or will that happen on arrival in Dublin (going) and on arrival at San Francisco (returning)? I am very concerned that we have enough time for the layover but if we don't have to change terminals or go through customs, it should be no problem. Help!
Use the LHR flight planner which explains what you have to do at LHR
http://www.heathrow.com/flight-connections
On your inbound flight (SFO-LHR-DUB) you will pass through immigration in London as the UK and Ireland are part of the Common Travel Area. If this is all on one ticket any luggage you have will be checked in at SFO and you collect it in Dublin. Easy.
On your outbound flight (DUB-LHR-SFO) once again use the above planner. You will not go through any border control at LHR, though there will be security. You check your luggage in Dublin and collect it in SFO. The US pre-clearance at Dublin does not apply as you are flying to London not the US.
The connection times are OK though don't get side tracked at LHR by all the shops!
First, please understand there are two separate things to think about: Passport Control (immigration) and customs (bags).
Is your flight from Heathrow to Dublin actually on Aer Lingus or a codeshare? United arrives at Terminal 2. Aer Lingus also operates at Terminal 2. But Aer Lingus codeshare flights are operated by British Airways out of Terminal 5. (If the flight number is three numbers, it's an Aer Lingus flight. If it has four numbers, probably starting with an 8 it's a codeshare on BA.) If your flight is operated by Aer Lingus, you should be okay. If operated by British Airways you probably won't have enough time.
As stated, you will go through immigration and security at Heathrow. You will also go through passport control in Dublin. (I've done this route a few times and have always had to go through passport control in Ireland from Heathrow.)
You don't go through customs until Ireland. 99% of the time it is just a walk through. You won't get stopped.
On your return, you will not have to go through Passport Control at Heathrow but will go through security. You will go through Passport Control and Customs at SFO.
If you are booked on one ticket from SFO to DUB and miss the connection, the airline has to put you on another flight. There are numerous ones all day long. If you are making your own connection, you are cutting it way too close. What if your arriving plane is late?