While it's difficult to know exactly how much corned beef is eaten on St. Patrick's day, this is considered more of an American tradition. In Ireland, it is not customary to each corned beef on the holiday.
The American tradition of eating corned beef on St. Patrick's Day likely originates from the fact that Irish immigrants to the U.S. were able to get cheaper corned beef in their new country. The traditional Irish St. Patrick's Day meal uses pork rather than beef. In most cases, Irish immigrants to America would have been unable to afford beef in Ireland and, seeing it as a luxury, were likely drawn to it thanks to its low price and wide availability in the U.S.