Dublin connections is a bit of a mess.
AFAIK everyone needs to go though an immigration check when connecting simply because UK and domestic departures aren't segregated from others.
Where you do the immigration check (new connections centre, old connections centre, or arrivals immigration) and whether you need to clear security depends on what gate you arrive at and what time. The first regular flight of the day arriving at T2 from a place that doesn't meet EU security standards is Emirates, so once that first Emirates arrives (11.20ish in the winter), anyone whose flight arrives at a gate that's not physically segregated from Emirates passengers is "dirty" from a security perspective and needs to go though security screening. There are barriers between gates 409/10 and 411/12 to keep part of the concourse permanently "clean".
Early one morning last week, I arrived on EI at gate 407 or 409 (gates in the area that becomes "dirty" when Emirates arrives). The announcement on the flight before we landed was out of date and said "connecting passengers go left just before immigration" (directing people to the old connections centre). As I exited the plane, the barriers to the left (separating the always-clean arrivals from the possibly-dirty ones) appeared to be open (passengers from my flight could get through the barriers to the new connections centre in the always-clean area). The old connections centre (to the left of arrivals immigration) was closed, with signs telling people to exit through arrivals immigration. An inconsistent and potentially confusing experience for connecting passengers!