I've used Aer LIngus through Dublin to get to Duessseldorf. It was very easy. We had a 1 hour 45 minute change time. I had time to find an ATM and get EURs, buy a cup of coffee. After getting off the plane in Dublin, we walked down a hallway, and at one point, there was a split and an airport employee directed us one way to the terminal for flights to Europe, and another direction for people going to Dublin baggage claim and flights to elsewhere in Ireland. I don't recall any passport check. You get that when you land in Berlin along with picking up your luggage and going through customs (just waking out the "nothing to declare" door).