Potatoes and cabbage pounded together and then fried in butter (Irish). “Col” is cole or cale, i.e. cabbage.
“About 1774 Isaac Sparks, the Irish comedian, founded in Long Acre a Colcannon Club.” —The Athenæum, January 20th, 1875.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Colcannon from Fact Monster: