Well, first you find an Irsh person and................, just kidding. If you are referring to what is called "Champ", you boil your spuds as ever while at the same time slowly heating spring onions or "scallions" as we call them in buttermilk. You can also use sweet milk if the thought of buttermilk puts you off. When the spuds are boiled are ready, you mash them and pour the the scallions and milk into the pot and whip them in. To serve, create a great mound out of the spuds and add a large dollop of butter on the top. A heart attack waiting to happen!
When I was a kid in Belfast, this was one of my most favourite meals, all by itself. Now I live in Canada and have introduced it to my family and a number of friends.
Hope this is what you wanted.
Alan