Interesting thread for me as was on a very long flight once JNB-AUH-DUB-CPN and as the tickets and baggage check were in-line, i was told by etihad one time to try to come through that T2 transfer point before immigration to use their lounge on the Dublin side. however, my last flight to Copenhagen was from terminal (T1). As the etihad lounge closed at 9am and their flight arrived around 8ish, there would have been no way to clear exit in T2 and return back through T1 security and hoof back to T2 etihad lounge ( my bag was checked through to my final SAS destination). I was stopped after turning left before immigration and the transit person was insisting that you couldn't transit from T2 to T1 at that point. I argued my case stating they are connected airside anyway so why not and the etihad lounge were closing soon and I needed to get there etc. after a number of phone calls, They finally relented and let me in, scanned my SAS boarding card, which brought up all my details, so its not a technical reason, and I got to the etihad lounge at about 10 minutes to 09:00, which is literally 20 steps or so from that point in time for a nice shower before traversing over to T1 to make the 11:20 flight. Someone later told me they don't allow cross T2/T1 transfer there in some agreement with the T1 shopping facilities, which sounds crazy to me. Maybe it's changed now as this was about 2 years ago.
The long and short is, you won't be able to get through without a boarding card for sure.