What is the best route to take while traveling all European countries? And best itinerary?

From Ireland I would go to the UK take a train from there to Paris France, Belgium, Netherlands, Germany, Denmark, Sweden, Germany again, Poland, Czech Republic, Austria, Switzerland, Italy. As for flying home I'm not sure whats best for you. For us its about a $1,000 cheaper to fly from Dusseldorf Germany - Amsterdam- Minneapolis, even though Amsterdam is closer and is a straight shot to home for us.