The name probably derives from 'cole', used at least since the 10th Century in England as a synonym for brassicas, and 'cannon' may mean 'to pound'. The exact name is known at least since 1801 in the Anglo-Irish wrtiter Maria Edgeworth's 'Moral Tales' "Forester..dined like a philosopher upon colcannon."
Original Receipt in 'A modern system of domestic cookery, or, The housekeeper's guide' by M Radcliffe. (Radcliffe 1822)Colcannon.
Boil potatoes and greens, or spinage, separately. Mash the potatoes, squeeze the greens dry, chop them quite fine, and mix them with the potatoes with a little butter, pepper, and salt. Put it into a mould, greasing it well first; let it stand in a hot oven for ten minutes.