It's March 17th, St. Patrick's Day, or as The Onion describes it,
the annual reinforcin' o' the stereotypes. The celebration of this day within the United States is, of course, rooted in the history of Irish immigration into the U.S. during the 18th and 19th centuries. St. Patrick's Day, along with a handful of other nationality/ethnicity based holidays, also is an indicator of the weird ways in which the peculiarities of U.S. history have shaped how we view ourselves and our pasts.
Up until the early-to-mid-20th century, it was common for immigrants to settle in geographically proscribed locations. In rural areas, this would include Irish settlers in the eastern mountains, French settlers in particular parts of Canada and the American South, and Norwegian settlers in portions of the midwest. Within urban areas, this often resulted in the creation of ethnic neighborhoods: Little Italies, Irish Shanty Towns, Chinatowns, etc. Regardless, the end result was that there were semi-insulated pockets of immigrants and their children and grandchildren living in the larger tapestry of their cities or regions. Within these pockets, traditional religious practices, customs, and language continued to be used, to the point that some members of these communities may die long before they had to integrate into society outside of the smaller community*.
Often, ethnic/national prejudices played a role in keeping these neighborhoods corporate. It wasn't always "white vs. minorities", Europeans have historically been very good at coming up with arbitrary and absurd hierarchies amongst themselves and using it to oppress each other as well**. Someone from Chinatown might have learned that you can't trust anyone outside the neighborhood, and someone from Little Italy might have learned that you don't rely on anyone who isn't Italian. This had many results, including the rise of organized crime within these communities, but it served to reinforce the identity and allegiances of people from them as well.
This had a variety of rather fascinating impacts on both these communities and the broader society, but it the one I want to discuss here is the fact that it often reinforced and reified an ethnic identity that was increasingly divorced from its geographic roots. People who were raised in neighborhoods populated by the descendants of immigrants from Ireland, Italy, China, Sweden, Korea, Germany, etc. might think of themselves as Irish, Italian, Korean, Swedish, and so on and so forth. These were typically people who had never been to these places, people who had only the most tenuous grasp on what the culture of those places was decades early (and no comprehension of what it was at that present time). It also appears to have resulted in the notion of a sub-nationality being strongly imprinted onto the American psyche. We are not simply Americans, or citizens of the United States. Many of us are what some folks refer to as hyphenated-Americans (Korean-American, Irish-American, etc.).
Interestingly, there are some nationalities that you don't often hear imprinted on-top of the American label (it's rare that one meets an English-American or a French-American, for example). While I am not certain, I suspect that this probably has alot to do with certain nations (primarily England, France, and Germany) being dominant during the colonial period. If you hold sway (or at least have enough in common with those who do that you can get by), you have little reason to signal your allegiance or have others label you. This would also explain why these labels are more important to those who are from groups that are or recently have been oppressed, as well as those who physically stand out.
At any rate, we have this phenomenon of a set of sub-cultures within the U.S. that are descended from other cultures in Europe, Africa, Asia, and the Pacific Islands, but which has since then evolved divergently from the parent culture. So, every March 17th we see a number of U.S. citizens, born and raised in north America (as were several generations before them) claiming to be Irish, but engaging in customs that likely would leave the modern Irish scratching their heads wondering just what the Hell these Yanks are doing.
Anthropologically, it's rather fascinating. But I suspect that, were I Irish, I would find it annoying.
*When I hear people today discuss immigrants from Latin America or Asia and their tendency to hold on to old cultural traits as if it were a new and dangerous trend, I always try to point out that this has always been the way of things, and that the ancestors of the people doing the complaining likely behaved in much the same way.
**To be fair, though, this is not unique to Europeans. The same phenomenon is well documented on pretty much every continent.