Epicurious says these are called "champ" in Ireland. Duncan agrees that this is a traditional method of mashed potatoing over there, but he is not certain that the name is accurate.
6 medium potatoes (any white potato will do)
1/4-1/3 c cream or half & half*
1/4-1/2 c milk
2-3 T real butter
4 scallions/spring onions/green onions/whatever you call them
salt & pepper
roughly chop the scallions, going about halfway up the green part. put them in a small saucepan with the cream and milk. bring to a simmer, cover, and turn off the heat. let steep while potatoes are cooking.
cut potatoes into one inch chunks, put in pot, cover with cold water. do not add more than a dash of salt to the water. cook on medium high until potatoes are soft enough to pierce easily with a fork.
drain potatoes well, and mash slightly. add the milk/onion mixture and mash to desired consistency. season to taste.
*I did not say this was going to be healthy. but if you must, you can leave out the cream and/or cut down the butter.