I learned how to make this traditional Irish dish when I lived in New Orleans. We were watching the St. Patrick's Day parade from my husband's boss' balcony on St. Charles Avenue in the Garden District. In the kitchen his wife was preparing Colcannon and she gave me a lesson. Her version (and mine) is as simple as can be--no frills.
Peel potatoes, quarter, and place in pot of boiling water. Cook over low heat, covered, for 20-30 minutes, or until tender; drain.
Remove outer leaves of cabbage and discard. Cut cabbage into fourths; wash well. Cut in half again crosswise. You may discard the core or you may chop it into small pieces. Place cabbage wedges into large pot of boiling salted water. Reduce heat, cover, and cook about 25 minutes or until done. Drain and put back into large pot. Keep warm.
With a potato masher, mash up the potatoes to a semi-lumpy stage. Add half the butter and salt, and all the milk, a little at a time. With an electric mixer, blend the potato mixture on medium-high speed until they are smooth.
Add the mashed potatoes to the drained cabbage in the pot. Blend well. Add the remaining butter and the leeks or green onions, if desired. Add remaining salt, to taste, and the pepper. Mix well and serve.