Originally Posted by
Ber2dca
There's definitely immigration at LHR when you connect to Ireland and arrive from the U.S. and there will be controls in Ireland again, too. Though in both places it's a pretty quick and cursory process usually. The UK authorities basically treat the Ireland flights like domestic UK flights whereas the Irish authorities treat them as international arrivals.
Bingo.
There will also be one additional step to be aware of: the passengers will have their photographs taken at security, and when boarding there will be check against the boarding pass name and photo taken at security to ensure the same person is boarding. For a first-timer this will seem very odd, but don't worry: it's done this way because departing international and UK/Ireland passengers are amalgamated so it's important to ensure that someone who's not been through UK immigration doesn't swap boarding passes with someone who has, potentially circumventing formalities.