Originally Posted by FlyerLX

Your DUB inbound is likely to arrive at non-Schengen B, Z or a remote stand. Your JMK departure is likely to depart from A, Schengen B or a remote stand.

As seen in this thread, the theory is that if you arrive clean, you can transfer without security check. I'm pretty sure ROI counts as clean because it's EU (although, just for info, plenty of non-EU countries are clean for transfer in DE too.) As your arrival is non-Schengen, you will go through passport control because you are entering Schengen at FRA.

The theory does not always translate into practice.

If you arrive on Z, you're in luck: just follow the signs to A (reasonably clear for FRA, amazingly) which take you through passport and down a big escalator to a huge duty free area where you can either spend all your money or just walk through to A. You see a confusing security check straight ahead of you but that's not for you. If you depart from A, just follow your signs to your gate; if you're departing from Schengen B, just go through the tunnel from A to B and follow the signs past the passport check at the other end, and go on to your gate. Don't go through passport control when you emerge from the tunnel as that takes you back out of Schengen (I think.)

If you arrive on B, however, you cannot transfer anywhere else without a security check, so you (as I understand) have no choice but to go through security. This is a case of airport logistics taking priority over an international agreement. Follow the signs as best you can, and do the same if you arrive on a remote stand.

FRA is a maze, and the staff are really not (as I have found regularly in recent times) particularly pleasant or helpful. Try to book via MUC in future.