Individuals can be Irish from their ethnic heritage of a common history of mores and folkways within the culture. It is a personal choice of how and to what degree one chooses to identify with the history of their ethnicity. It could be as recent as a year or longer as dictated by a family tree that has many limbs and branches. And for some just for a special day. But whatever it is, as my staunch Pennsylvania Dutch (German) mother admonishes, "if you observe or celebrate St. Patrick's Day. have fun and put some green food coloring in my Vodka".