Mar 3, 2009 Stephanie Jolly , Source: Suite101.com

While many North Americans associate corned beef and cabbage with Ireland, this popular St Patrick’s Day meal has roots in America, and is not traditional Irish food.

Corned beef, a salt-cured brisket, was traditionally packed and stored in barrels with coarse grains, or “corns” of salt. One of the earliest references to corned beef appears in the 12th century Gaelic poem Aislinge Meic Conglinne, where it references a dainty, gluttonous indulgence. By the 17th century, salting beef had become a major industry for Irish port cities of Cork and Dublin, where Irish beef was cured and exported to France, England and later to America.

Traditional Irish Recipes Contain Salt Pork Instead of Corned Beef
With the majority of Irish beef being exported, beef was an expensive source of protein and unavailable to the majority of Irish citizens. Cows, if owned at all, were raised predominately for their dairy products, from which butter, cheese and cream could be obtained, and were only slaughtered when they were no longer good for milking. Sheep were raised as a source of wool and hogs and pigs were one of the only livestock species raised by the peasantry for consumption.
Salt pork and bacon, therefore, became the commonly consumed meat protein of Irish tables. In Feast and Famine, Leslie Clarkson writes that “fat from bacon supplemented the lack of fat in the farmhouse diet” and Sir Charles Cameron states that he does “not know of any country in the world where so much bacon and cabbage is eaten.” Even today corned beef and cabbage appears infrequently in Irish pubs and restaurants, except for those in heavily touristed areas, and is much more likely to be replaced its traditional counterpart – an Irish stew with cabbage, leeks, and a bacon joint.

Corned Beef & Cabbage Eaten by Irish Immigrants After Arriving in America
After the Irish potato blight, or Great Famine, of the mid-19th century brought hundreds of Irish emigrants to the shores of America, the newly immigrated Irish Americans found corned beef to be both more accessible and more affordable than it was in Ireland. Both corned beef and cabbage were ingredients of the lower working class, and their popularity among the Irish population likely had little to do with similarities to the food of Ireland and more to due with the relatively inexpensive nature of salt cured beef and green cabbage.
For several decades following the Irish immigration, St Patrick’s Day was celebrated with music, crafts and revelry but banquets, while lavish, contained a scarcity of traditional Irish cuisine. However by the 1920s, corned beef and cabbage came to have an association with Irish American cooking, according to Hasia Diner in Hungering for America: Italian, Irish and Jewish Foodways in the Age of Migration and joined Irish bacon and greens as a food reminiscent of Ireland.

Corned Beef’s Association with St Patrick’s Day Has Irish American Origins
While both salted beef and green cabbage have historic connections with Ireland, the ritual of serving corned beef and cabbage for St Patrick’s Day is exclusively an Irish American tradition. The scarcity and high price of beef in Ireland prevented it from being consumed by the majority of the Irish peasantry until arriving in America, where corned brisket and cabbage were cheap and readily available to the poor. As the stigma of eating working class food faded and the celebration of Irish ancestry grew in popularity, corned

And from Foodtimeline.com, we have:

“Corned beef
While the process of preserving meat with salt is ancient, food historians tell us corned beef (preserving beef with “corns” or large grains of salt) originated in Medieval Europe. The Oxford English Dictionary traces the first use of the word corn, meaning “small hard particle, a grain, as of sand or salt,” in print to 888. The term “corned beef” dates to 1621.
“Emphasizing its long history in the Irish diet, Regina Sexton…points out that a similar product is mentioned in the 11th-century Irish text Aislinge meic Con Glinne many wonderful provisions, pieces of every palatable food…full without fault, perpetual joints of corned beef’. She adds that corned beef has a particular regional association with Cork City. From the late 17th century until 1825, the beef-curing industry was the biggest and most important asset to the city. In this period Cork exported vast quantities of cured beef to Britain, Europe, America, Newfoundland, and the W. Indies. During the Napoleonic wars the British army was supplied principally with corned beef which was cured in and exported from the port of Cork.”
—Oxford Companion to Food by Alan Davidson, [Oxford University Press:Oxford] 1999 (page 218)

Corned beef was very popular in colonial America because it was an economical and effective way to preserve meat. The following corning directions are from The Virginia House-Wife by Mary Randolph, 1824, pages 22-23:
“To corn beef in hot weather
Take a piece of thin brisket or plate, cut out the ribs nicely, rub it on both sides well with two large spoonsful of pounded salt-petre; pour on it a gill of molasses and a quart of salt; rub them both in; put it in a vessel just large enough to hold it, but not tight, for the bloody brine must run off as it makes, or the meat will spoil. Let it be well covered top, bottom, and sides, with the molasses and salt. In four days you may boil it, tied up in a cloth, with the salt, &c. about it: when done, take the skin off nicely, and serve it up. If you have an ice-house or refrigerator, it will be best to keep it there.–A fillet or breast of veal, and a leg or rack of mutton, are excellent done in the same way.” “Some people wonder about the shared culinary/cultural heritage of the Irish and Jewish peoples when it comes to corned beef. The practice of curing meat for preservation purposes certainly dates back to ancient times. The use of salt was adopted/adapted by many peoples and cultures, and was widely used during the Middle Ages. Evidence suggests that both Irish and Jewish cooks were making corned (salt) beef independently, long before they met in New York.

“Corned beef comes in two versions: The Jewish special on rye, or the traditional Irish boiled dinner, aka New England boiled dinner. Tonight should be the big night for the Irish version.”
—Boiled dinner, The Boston Globe, March 15, 1990 (p.3)