Originally Posted by
Gagravarr
If you're flying from the USA to Ireland via Heathrow, on a through ticket, then on arrival into Heathrow you'll just go through security. You won't clear customs, you won't clear immigration (unless you decide to pop out for a smoke!), but the onward airline will want to see a passport to check you have the right passport/visa to enter Ireland
As noted this is absolutely not correct. International passengers arriving at LHR and transiting on to DUB (or indeed any UK airport) go through immigration (it is the same queue for UK and DUB connections at the border) and security. You would have your photo taken as well as part of this process and this is reconciled with you at the gate when boarding.