From T2 arrivals, you definitely do have to clear security and immigration (separate immigration to the arriving passengers, though) again to make your connection whether in T2 or T1.

Keep to the extreme left when you get to the arrivals immigration checks - the transfer path leads you around a corner, to the left of the normal arrivals checks, and you pass immigration in this separate line, clearing security just beyond. At that point, you can walk either to T2 or to T1.

Sort of surprised that from T1 you don't have to do this - but maybe it's because all the longhauls are operated from T2 (not that Ireland is in Schengen or anything!)

Borrowing this scan of the transfer map from Cara magazine from a user on the EI board: