Are you traveling on your own or with friends?

Personally, if I was on my own, I wouldn't do Ireland. I've been to Ireland several times before, and I think one of the best ways to get around is renting a car. It allows for maximum flexibility in stopping in little towns and exploring the countryside. Renting a car on your own is a hassle and isn't much fun. You can certainly travel through Ireland by public transportation, but it isn't nearly as flexible.

In terms of Benelux or Scandinavia....both are good options. I'd probably pick Scandinavia, both because Iceland is a fun spot for a stopover, and because summer is an ideal time to take advantage of the extended daylight up north.