The recipe for Colcannon we used for years had leeks and when leeks became harder and harder to find, I stopped making it. I have since lost the recipe but I can give you the gist. I tried looking up a similar recipe and discovered that there are so many variations that I was stunned. I think I am going to pull this out again and try making it with onions instead of leeks.
This one from allrecipes.com is quite similar to what I used to make.
1 pound cabbage
1 pound potatoes
1 cup milk
salt and pepper to taste
1 pinch ground mace
1/2 cup butter
In a large saucepan, boil cabbage until tender; remove and chop or blend well. Set aside and keep warm. Boil potatoes until tender. Remove from heat and drain.
Chop leeks, green parts as well as white, and simmer them in just enough milk to cover, until they are soft.
Season and mash potatoes well. Stir in cooked leeks and milk. Blend in the cabbage and heat until the whole is a pale green fluff. Make a well in the center and pour in the melted butter. Mix well.
Note: if you use leeks, wash them very well. Do to their design leeks gather dirt in between their "leaves" and can ruin a great dish.