Just to add having been through the new version of the 200 gates at DUB a few times in the past few weeks (not doing a B2B) I haven't seen any evidence of a way to double back to the gate area and as
c-w-s hints at above suggests either a door was open in error or similar if there was a suggestion this remained possible (I couldn't quite see the reference up-thread).
dca100 it sounds like you went to the end of the baggage hall and used the Flight Connections channel; what most people would advise to do in this context is exit the baggage hall entirely and go landside, then re-enter airside via the "normal" fast track security entrance, which tends to be quick.
Still doesn't resolve the passport queue issue, for which there is no easy solution and does indeed mean those with a low risk tolerance probably are best off not attempting the B2B route on BA at DUB (in my opinion anyway).
Edited to add: From some poking around on the Fingal County Council planning portal I located the planning application and supporting materials, which includes the background/rationale document submitted to support the application, which may be of interest.
Pier 2 planning application