Sorry to post on an old thread, my questions are a little more specific to holding a USA passport in case it mattered.

As of posting Oct 2016 Is it still true that on a USA-LHR->DUB itin, one needs to get a passport stamp into the UK at heathrow, and then another stamp at dublin?

Further, How much time does this process take at heathrow? Will 75 Minutes be enough at LHR?

Thanks