It is strange that an obscure feast day, beginning in some of the obscure areas that St. Patrick was revered, then spreading to the rest of the UK, then to the rest of the world. St. Patrick's day is celebrated in such far-flung areas as Taiwan, along with most of the rest of the globe you can think of. Everyone in the world seems to be Irish on this day. Why would this be? In my opinion there are a few characteristics of this holiday that have encouraged it to spread worldwide.
First, St. Patrick's day is no longer really about St. Patrick or what he stood for at all. In fact, very few people know much about him or who he was, when he lived, or why he has a holiday named after him. It is said to have origins that date to the day he died, and have imagery that showed his talent for using pagan symbols such as clover and green to promote and substitute for Christian icons such as the cross, replacing their pagan gods and goddeses with similar biblical ones such as the Virgin Mary (replacing Dianna, goddess of fertility and childbirth). But the fact that these things aren't stressed or even known probably plays to the holiday's advantage in a way. Only a sixth of the world is Catholic and even fewer of those are devout enough to think that worshipping a Saint equals a good time. This has enabled the holiday to fit in with even non-Christian religions.
Another reason paralells the story of the Irish in America. Did you know that that traditional corned beef and cabbage as a meal for this day was solidified by Irish immigrants in the early 1800's and not back in Ireland? Fist of all, beef was their most important export back in the mother country, so they rarely ate it - it was much too dear. They used salt pork instead for main meals. And so much for potato pancakes as an authentic mainland Irish tradition, because job-seekers left because of the great potato famine in the first place. But when the Irish got to the US they celebrated their comparatively new prosperity, and did it with food, still one of the only things they could splurge on since it was a while before they shook off poverty completely. And beef and potatoes featured squarely in the center of their new traditions. Their food was tasty to the non-Irish as well.
And one thing that most people have alwways believed about the Irish - they know how to have fun. They seem to be able to have more fun during one pub night then some of the rest of us have in our entire lives. That attracts a lot attention and envy. So the guy who a few decades earlier might have had a "No Irish" sign on his business may have been eventually tempted into one of their pubs, if not by the food served but to see what all the merriment was about.
Irish didn't stay a persecuted minority for long (well no more than a hundred years or so). The prosperity of the early twentieth century raised their value because new and prospering, non-farm companies needed labor. And many of the immigrants had become "bi-cultural," meaning you could act Irish at home and just like anybody else when you went to apply for a job. But with this new-found success and acceptance, it only continued to bring more high-status attention to the things that they had going for them. In the large cities, where St. Patrick's day, it has spread, and is celebrated by parades, resulting in parallel activities in every obscure town there could possibly be.
It is unfortunate that the Irish received so much persecution when they got to this country. But one thing this did was result in a tight-knit community, one that is likely to preserve traditions like music and dancing. And in general, the cultures that have strong families and friendships, ultimately become respected, as did the Italians who had similar problems at first.
Because of their strengths and initial hardships, anyone would be proud to occasionally identify with this plucky, pick-pick-yourself-off the ground, family people that all know how to have a good time. Another thing that is common to all popular holidays is good icons. It has the green, the leprechauns and the shamrocks (however they originated - most don't even ask), just like Christmas and Easter, which have also spread around the globe, having the universally-popular Christmas tree and the Easter Bunny as flexible iconography.
So because of the Irish themselves and characteristics of the holiday, it has become wildly popular, unfortunately for those that admire his unique and amazing achievements among the Pagans in Ireland, leaving St. Patrick somewhat behind. Unless clearly ethnically different, people are always telling you they are 1/8 Irish, or 1/16 Irish, or making it up if it isn't true. The kids love it for the pinching for not wearing green; the adults love it for the revelry and green beer. But everyone loves St. Patrick's day.