THE feast of Ireland’s patron saint has always been an occasion for saluting the beautiful land “where the praties grow,” but it’s also a time to look again at the disaster that established around the world the Irish communities that today celebrate St. Patrick’s Day: the Great Potato Famine of 1845-6. In its wake, the Irish left the old country, with more than half a million settling in United States. The famine and the migrations changed Irish and American history, of course, but they drastically changed Britain too. Americans may think of the disease that destroyed Ireland’s potato crops, late...