Every year around St. Patrick's
day in the U.S. the grocery stores start putting corned beef brisket on sale and on display, and restaurants and pubs add corned beef and cabbage to their menus as the Irish entree. Unfortunately, corned beef and cabbage, even when accompanied by potatoes, is more American (or Germanic) than Irish; we'd do better to celebrate Irish cuisine with salmon or colcannon, a dish of potatoes and cabbage (that's even better made with kale).
As much as I like corned beef, I'm here to tell you that it's not really very Irish, though it is very American. Historically, the Irish raised pigs for meat, and beef for milk. If you butchered a cow, you did it in later October or early November, at Samain, and by March that meat, even if you cured it by corning it, was gone. Pork was a staple of the Irish diet, particularly bacon. When Irish immigrants arrived in Boston and New York, their beloved Irish bacon was not available; what was available was corned beef, thanks to Jewish delis and butchers in New York, and the New England Boiled Dinner popularized by German immigrants to Massachusetts and other New England states. The corn in the phrase corned beef refers to salt-curing, or brining. The corn refers to the large grains of salt used in curing the beef. The meat was placed in a crock and liberally covered with large grains (or corns) of salt. Etymologically, corn comes from the Germanic root kurnam, used to refer to small seeds or kernals, cognate with kernal and with Latin grain. Corned beef, traditionally made with a brisket of beef in Eastern European and Jewish traditions, is fairly simple to do at home.
The Irish immigrants, unable to locate or in some cases afford the distinctively different Irish bacon,
traditionally served with cabbage and potatoes, possibly with carrots or other vegetables turned to corned beef. Traditionally, beef in Ireland was a luxury; in the middle ages, cows were prized for their milk. Dairy products were so important in medieval Irish diets that cheeses and similar milk products were called "white meats." In the eleventh century medieval Irish satire Aislinge Meic Con Glinne/The Vision of MacConglinne, MacConglinne attempts to entice a "demon of gluttony" to exit an abbot by preparing a magnificent feast that includes "And he called for juicy old bacon, and tender corned-beef, and full-fleshed wether, and honey in the comb, and English salt on a beautiful polished dish of white silver, along with four perfectly straight white hazel spits to support the joints." This is very clearly an over-the-top outrageous feast, and the use of corned beef in the feast points up that it was considered extravagant.
Pork was by far the more common meat, even after the English conquest, since what beef there was was exported to England (especially during the Napoleanic wars), leaving potatoes, fish, pork and cabbage for the native Irish. You'll find Irish families and restaurants even now having gammon, or a roast joint of pork, as a family dinner. The Irish kept emigrating to America, bringing Irish cuisine and traditions with them but of necessity modifying them to suit the new land.
The very first St. Patricks Day parade was held in Boston in 1737; what was until very recently a solemn day of church attendance in Ireland, followed by a Sunday dinner with the family, very quickly became an opportunity to celebrate ethnic pride as St. Patrick's Day parades took place a few years later in New York, and quickly became an annual tradition all over America. At the same time, in part because of tourism from descendants of Irish emigrants returning to Ireland, St. Patrick's day is becoming more of a secular tradition in Ireland. Though I suspect the inclusion of corned beef on St. Patrick's Day menus there is a nod towards their American guests; Irish cuisine today, freed from the burden of export, features local and fresh ingredients, and while pork and potato are still important ingredients, so are the amazing local mackerel, prawns, lobster and salmon, and beef as well as Irish cheeses, and incredible locally grown produce, and of course, the lamb that's so much a part of Irish Stew.