The reason mashed potatoes are called champ is the word "champ" comes from the Irish Gaelic word "bruitin" which means mash.
Colcannon and champ are both favorite dishes of Irish mashed potatoes.
And they are both similar, although champ often tends to feature spring onions, or scallions alone.
Colcannon adds cabbage, leeks or kale.
Champ is basically mashed potatoes with chopped spring onions (scallions) and milk.
Colcannon is also Champ with the addition of cabbage and sometimes some herbs.
The word colcannon is derived from the Gaelic term cal ceannann, which means "white-headed cabbage".