Colcannon sounds like a wonderful way to spice up regular mashed potatoes! Onions and kale are both packed with flavor, so they would add a lot to the taste of the dish. Green onions are particularly pungent!
I probably wouldn’t put the extra margarine on top. I actually like using just a touch of margarine in all of my recipes, because I like to keep my fat and grease intake to a minimum. Also, too much can hide the flavor of what’s underneath. On the same note, I would use just a small dash of salt as well.