Flying from US to Germany via Aer Lingus, both legs, connecting flight is in the same Dublin terminal (2) and arrival is in early morning (6 AM). Is 1.5 hours enough time to get through immigration/customs to the gate? Three passengers, 2 Americans, 1 European. I'm guessing we're okay, since it's not a crazy bustling airport, same airline, early morning, but just want to make sure.
For this specific connection there is no need to go through immigration, customs or even security in Dublin - you simply follow the "Flight Connections" sign when you land and you will be in the departure area for your connecting flight.
Given this, 1.5 hours is plenty of time, presuming your inbound flight is on time.
If your inbound flight is late, then because you're on a single ticket it'll be the responsibility of the airline to book you on a later flight at no cost to you.
If you're flying the same route on the return then it's a different story. Passengers from Dublin to the US pass through US immigration/customs/security in Dublin before boarding the plane, so you will require additional time to complete those formalities.
Contrary to what the other answer states, transit at Dublin does involve a special transit immigration check by the Irish INIS.
That said, should this cause you to miss your flight, you will be re-booked onto the next flight, since the trip is in one booking