No, the inbound aircraft uses a special route by gate 23, where you can go landside without passing the UK Border. The passageway ends up behind the back of UK Border so you still exit via international baggage reclaim: this is necessary since in theory at least there can be an HMRC inspection on Dublin passengers (snowball's chance however). There are some nationalities who are suppose to declare their arrival in the UK (visa regulations between UK and Ireland are only centralised for a few countries), so they could choose to go out via the UK Border - if they persuade the HAL agent to let them through, however it would be easier to use the red telephone in the customs area since the Border Force work in that area too. Very few people need to do this however.