I still don't understand why biometrics are required to go to DUB if passengers voluntarily present their (British or Irish) passport or some other document which guarantees entry to Ireland.
A bit of speculation here, but I think it may be less to do with whether entry to Ireland is guaranteed and more to do with the passenger's current immigration status in the UK. The biometrics step is intended to ensure that the passenger is
already "inside the UK" for immigration purposes before they board a flight whose premise is that all passengers on board are already "inside" (regardless of whether it's then going to a UK or a ROI destination).
Otherwise the passengers on board the LHR-DUB could include someone who holds a British passport, who has arrived at LHR from outside the CTA, and who has not been admitted to the UK by an immigration official. On arrival at DUB, they would not be examined by the ROI because the ROI assumes that everyone on the flight was already "inside the UK" and the ROI then doesn't do an additional check of those holding British passports. The result would be that the passenger would have entered the ROI from outside the CTA without actually being examined or admitted by the immigration force of either country. Having an airline/airport staff member look at your passport
en route isn't quite the same thing.
All of this makes more sense for domestic flights, but as in an earlier thread: the CTA represents a great deal of pragmatic compromise, but you have to do all the bits of the pragmatic compromise to make it work.