you do have a choice with a DUB-INT connection.

You will arrive in to A23/A22. You can either turn right, show your onwards bp to the Heathrow agents, and then go along to flight connections where you have a bp check and go up to security to get in to the airside departures area. Alternatively you take the DUB arrivals corridor opposite A23, this will bypass immigration but you exit via the international baggage hall (nominally there is customs), then out landside and up to floor 3 for departures and through security to get to airside departures.

Normally I do the latter. However, if you are stepping off the plane with 45 or less to go I would strongly advice going through connections as you will get to the point where you scan you bp and meet conformance quicker than if you went landside.