With an overnight stop may be worth looking at separate tickets if the long haul flight is cheaper ex AMS than DUB.
Less likely though. I recently looked at an DUS-AMS-LAX return in C class. It was €1900 cheaper from DUS than direct from AMS (same flight, same dates).
The last time I wanted to make an overnight (with KLM) I had to do it by phone through their reservations department. But that was a few years back.